If you can, I'd probably go with a 5830. The 256-bit memory bus really helps give a performance advantage over the 5770. I know that while I still had my 1280x1024 monitor, the 5770 I have right now could max most of my games out, but just barely. I'm not sure how it would have handled something like Crysis, and high amounts of AA would have probably stopped it in its tracks.
Your PSU can easily handle a single 5830, and it would be a great match for your Athlon II 645. That's a really nice system you have there, by the way... good job on that build. Looks like it's a solid all-around performer.
Sounds good. I made a mistake in the post. I mean't a 6850, they are coincidentally about the same price right now from the site I'd order from. Think it can handle it. Thanks btw for the system complement, appreciate it