As an atheist, I agree with the general spirit of your argument, but I'll offer some critiques just so you can think about sharpening it up. It looks logically valid, but, of course, that doesn't necessarily make it nomologically sound. What complicates the argument is the cryptic meaning of some of the phrases, but since this is a larger problem present in all natural languages, I won't dwell on it too much. Nevertheless, you should watch out for the dual use of "unproven" and "unable to be proven" in the second premise, where you only talk about what is "not proven," which leaves open the possibility that believing in something that can be proven is logical, thereby ruining the argument. You highlight this distinction in all the other premises except this one, so it's not a big deal, but you should be consistent throughout.

It's easy to agree with the first premise. The second premise is the most "explosive," if you will. There are lots of implications and consequences in that statement. I'm not sure how I feel about it at first glance; it's something that requires much thought and analysis. String theory - the leading candidate for the Theory of Everything in physics right now - has also not been proven empirically, but I wouldn't say that it's illogical to believe in String theory. Why? Because there are plenty of great reasons to legitimize String theory, among them the explanation for the weakness of gravitational effects in our universe. The greater point is that even if something has not been proven outright, there may be other sufficient and redeeming qualities that an idea may have to qualify as logical or reasonable. Just from a cursory overview of the argument, the second premise seems to be the Achilles Heel. But again, include unprovable and this issue goes away.

Third premise....seems all-right. Nothing polemical there, if we ignore the problems of natural languages I mentioned above. Otherwise there are big problems, like what exactly do you mean by 'valid worldview'? You don't define this. Fourth premise....could suffer from the problems I talked about in the second paragraph, but doesn't because you made sure to include both unproven and unprovable. 

I think that depends on what you mean by "believe." If you consider it the most likely explanation, then that may not be illogical. If you assume it is true and base your beliefs on that, then that would be illogical. You must be willing to consider the possibility of being wrong. Personally, I'd agree that it's the best candidate, and likely the truest theory we have, but it could very well be flawed, so I don't assume it's true.

Yes, once willing to consider the possibility of being wrong, it is not illogical to believe that String theory is right. I myself don't believe in it outright, but I do recognize its value. I see nothing wrong, however, with people who do fully believe in it while holding certain reservations. Again, there are plenty of good reasons to believe in String theory even though it may not be right.

I have no qualms about adding to the argument, I wrote this up in about 5 minutes so there were bound to be blunders amass.

Thanks for the pointers though.

Might be me. I haven't been in a humorous mood lately. Also, a lot is lost in text only communication, so it can be hard to tell sometimes.

Then you feel it is rational to believe in all gods, pixies and unicorns?  If you DO have a cut-off, you are exersizing a reason that is "best". You are inferring to the best explanation, but stopping at a convinient point for yourself. The Smoothie God.

Depends on what you mean by "best." I notice you said you don't feel that what is rational is best. The truth may not feel good, but logic is "the best" for determining truth, not feelings. But it seems you would question whether it's best to know the truth. Beliefs inform your actions. Is it not benefical to have as many true beliefs and as few false beliefs as possible? The truth can be hard to accept or deal with, but doing so is better than living in denial.

Being able to accept and deal with things is a sign of maturity. Inability, a sign of immaturity. I'm not accusing you of being immature (at least not yet), but think on that.

How is it beneficial to have few false beliefs, and many true beliefs? Well, many true beliefs constitutes knowledge & information. The more the better. Not having many true beliefs constitutes ignorance, as does having many false beliefs. Surely you don't think ignorance is a good thing?

As I mentioned, your beliefs inform your actions. If your beliefs are flawed, your actions will ultimately be less than desirable in some way. They may be counter-productive/self-defeating, or be detrimental to others, or may allow you to be taken advantage of, etc. Whatever your goals, being better informed makes you more capable of achieving them.

If you think your beliefs don't matter, I would suggest that you haven't thought about it very much.

Perhaps you believe in belief, in which case I'd recommend you read Daniel Dennett's "Breaking the Spell: Religion As a Natural Phenomenon."

I don't think you can make a purely deductive argument for non-belief in a God, except for inconsistent or inherently self-contradictory versions.

You need at some point to refer to evidence (or lack of evidence) for the propositions which you are applying the deductive arguments to.

It is valid to deduce that belief in the actual existence of something for which there is little or no empirical evidence cannot justified on other than emotional or psychological grounds.

It is important to distinguish, IMHO, between a position that is not logically justified in terms of other facts, and one which is actually explicitly in conflict with the facts. I would restrict the labels 'illogical' and 'irrational' to beliefs which have strong evidence against them. If there is no evidence either way, I would prefer 'not logical' or 'not rational'. Many, probably most, of the things which give life its meaning and happiness are not 'rational' - we indulge in many activities because it makes us 'feels good', ie encourages various positive feelings. It is entirely rational to do things which make us feel 'happier', as long as they are not likely to cause us trouble later.

This is why I think it may be rational to hold a belief that is not empirically supported, as long as it is not explicitly in conflict with 'reality', if it fits in with other ideas and helps provide a framework for other thoughts, or is found 'useful', even comforting, whatever. I would be uncomfortable with someone who insists that their position is more than this, that this mental convenience and comfort corresponds to some solid external reality.

The risk with holding too strongly to beliefs that may not correspond to reality is that you may find yourself making poor decisions when faced with new circumstances. It surely is more likely that you will make poor decisions if they are based on factually incorrect ideas.