Assuming you want a new (as in not used), i would say go for the 5900XT... The leadtek version is nice because a lot of them come with 2.2ns ram (for that extra memory OC
For around 200 flat you can get a Radeon 9800 Pro which should give you some decent performance compared to today's cards. 5900 XT or 9800 Pro, they're both good cards, but out of all cards under 200, those are the ones you want to look at for some good performance.
Oh and your PC has enough power to get the most out of both of these cards.