Go for the ATI 9800 Pro's since they are much better than the Nvidia 5900 and even 5950. Many benchmarks already prove this. But then they are just benchmarks. But I do hear alot of people are saying that the 9800 Pro is a very good card.
You might be lucky to find one within your current budget range since the new generation of cards are coming out, the prices of the 'used-to-be' King of graphics are dropping considerably.