I'd go further and suggest that a lot of atheists would echo the hope that there is some kind of fabulous immortality beyond the grave, it's just that the concept is, upon examination, absurd.
No thanks.
If you argue from a base mechanics point of view (e.g. Your brain is basically a hard drive with n bits of storage an thus can only enter 2^n states) then you're assuming that in this hypothetical afterlife your physical limitations are maintained. Since your basic assumptions about the universe have been demonstrated to be wrong, assuming this particular assumption will be correct is drawing a very long bow. And the initial assumption is very much in doubt.
To my mind an atheist is someone who has yet to encounter a plausible religion, not (a) someone who defines their beliefs in opposition to one religion (e.g. I do not believ heaven is a green pasture populated by angels playing harps and dead Christians tending sheep) or (b) someone who is closed to the possibility that a plausible religion might exist.
That's ambitious of you.
>To my mind an atheist is someone who has yet to encounter a plausible religion
In principle, were a plausible religion to present itself, I would weigh it on the evidence. In the real world, the vast number of religions, none of which are plausible, combined with the demonstrated ability of science to explain the universe through natural causes, leaves no doubt in my mind that there is no supernatural creator.
As for an afterlife, the complete and utter lack of evidence or argument in favor of it consigns it to the realm of all other fairy tales. As such, I treat it as a thought experiment more than as a proposed state of the Universe. If there is an afterlife, either it's eternal or it isn't. If it is, it is boring beyond belief. If it isn't, then what have you "gained"? Sure, it'd be nice to have a few thousand years to think, but I'd far prefer telekinesis and control over the flow of time. Since the propositions of my having superpowers and my consciousness enduring after my neurons stop firing have equal chance of coming true, I'll stick with the fantasies that tickle my pickle.
Your a) scenario is easily dismissed: I think all religions are equally silly; some have just been around long enough and are familiar enough for their silliness to become engrained. As for b) I am not closed to the possibility in principle. As I said, I simply view it as vanishingly unlikely. I have a better chance of winning the lottery three times in a row than of any religion predicated upon the supernatural being "true".
You say "an atheist is someone who has yet to encounter a plausible religion". That implies that we're waiting for the right one, like a home buyer who's only been shown hovels. That's simply not the case. I am not, in practice, open to the possibility of a god; I think the whole notion is absurd. In the same vein, I am not open to the possibility of homeopathy being effective treatment for anything other than thirst. Certainly, were science to show that it was, in fact, effective, I would change my mind. That does not imply that I'm waiting to find the right kind of water to cure fibromyalgia.
PS: To quickly address your point:
>The idea that there are finite number of possible thoughts is highly improbable given the number of things there aren't finite numbers of.
I never meant to imply there were a finite number of things. Hell, you could count for all eternity and never run out of numbers. That does not imply there are an infinite number of things worth thinking about.
If so, then one would only need to demonstrate a sufficiently impressive failure of current scientific understanding to account for some events that would appear to be caused by his actions, in order to push what would be considered a plausible religion.
I might be mistaken about the use of the word "plausible", but I think it is not necessarily connected with evidence but with human perception. Of course rational people would only find plausible explanations that are backed with evidence, but even the most hardcore rationalists are subject to their own psychology and emotions.
My argument is that in a (completely unlike, hypothetical) scenario like the one I describe, most people would likely find plausible what that someone would claim (if it did not sound completely stupid) not because it would be the reasonable thing to do but because psychologically those acts would feel too imposing, if science could not explain them at the time.