Frankly speaking, anything faster than a 2000+ CPU shows similar percentage performance gains/decreases across the Radeon 9500/9800 series. So I wouldn't consider you CPU-limited in that sense with any of those cards ... but some of the newer games like to chew up CPU cycles, so that's an issue to consider.
As always, it comes down to what you want and can afford. Personally, I don't see the point in spending so much for the top of the line when the mid-range performs almost as well. It's just one opinion, but I think you would do best with a Radeon 9500 Pro and a newer CPU (2500+ and up) that supports a 333Mhz front-side bus (run sync with your memory). Those two together would cost the same as a 9700 Pro by itself and would give you the best overall performance increase.
But even with your current processor, I would think something in the 9500 Pro - 9700 Pro range would be best price/performance wise (e.g. 9700 nonpro). If you want to go for the high-end, the 9800 Pro is likely to be cheaper than the 5900 and be about equivalent in performance (some reviews have given the 5900 a commanding lead, like Toms and Anandtech, but others like HardOCP and extremetech have shown more of a wash).
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