Wrong question. You are here. What are you going to do about it?
"What should I do?"
Wrong question. You will do what you will do, ain't no "should" about it.
"What will I do?"
Now we're getting somewhere. We can answer this kind of question scientifically. By observing how people have acted in the past, we can predict how people will act in the future.
So far as I have observed, people try to do things that they will feel good about having done. For example, if i find something that makes me feel happy for having done it, i will tend to do it more. More generally we look for classes of things that make us happy, and explore the limits of those classes in search of things that will make use even happier.
What will you do?
Answer: you will ask "What will make me happy?"
and that usually leads to all sorts of silly questions about meaning and morality. Now you've read this blog entry you can get past all that stuff and find some useful answers :-P
A funny thing is that what makes people happy is often the act of making other people happy. It's not the only thing that causes happiness, pleasures of the flesh cause a small amount of happiness, and flow experiences cause considerably more, so this isn't a circular answer.
All rather utilitarian really. Not in the sense "you should be utilitarian", but rather "you are utilitarian (you just haven't realized it yet)".
By the way, i haven't mentioned infinity or immortality. Infinity is just a big rug to sweep fuzzy thinking under... life will have meaning an infinite amount of time into the future, life is given meaning by the infinite glory of God, immortality will give us time to scour the universe for answers, a finite life is worthless, a finite universe meaningless. Bollocks. Each moment of happiness is good of itself.