ATI x series hd card purchase question

I got a 3 GHz PC. 2 Gigs of memory. I am looking into a HD video card, I would like some suggestions on how to choose one, and which is better.

I am not too computer literate, I think a better card would have a faster processor and faster\more memory. Most info sheets I have looked at online do not say the card's processor speed, just a bunch of info that I still have yet to learn about. If I go down to the local CompUSA, they do not have too much in the inventory for me to look at the boxes, which have a bit more info.

I was looking into tha ATI X-series of HD cards, but maybe there are better choices?