With AMD/ATI cards, the important numbers are the last 3, the first is just the generation and doesn't see that big a change unless it's like a 2+ difference.

I.E. 5870 and 6870 are nearly identical in performance; 7870 would be a bigger step up from 5870.

But basically, for gaming performance, you really don't ever want to go lower than x770. The x450 range like what you got is for internet browsing, watching movies, etc.
Ideally, you'd want something from the newest 7xxx series, x770 or better. So, 7770, 7850, 7870, 7950, etc.