Regarding Crysis, read this:http://uk.pc.ign.com/articles/762/762201p1.html
So it looks like your PC will be fine for this game. Remember that Crysis supports Direct X 10 but it doesn't require it. You can play it fine with Direct X 9.
As for upgrading for Direct X 10, sure you'll need to do it eventually but I don't see any real rush. You'll need both a copy of Vista and a new graphics card, neither of which are particularly cheap.