AMD Route:
Best bang for the buck at the moment is probably an Athlon XP 2500+, or for a bit more a 2800+, paired to an nForce 2 based motherboard. The Asus A7N8X or A7N8X-Deluxe would be good choices. The Deluxe has better onboard sound, Firewire, Serial ATA, and dual network built on. Since you have a very good soundcard, you might want to save a bit of money and go with the A7N8X, which lacks the Firewire and Serial ATA. It still has onboard network, but not two NICs like the Deluxe.
The 2500+ and 2800+ are both Barton core Athlon XPs, which both have 512kB of L2 cache and a 333 MHz FSB. This means you could reuse your PC2700 memory. The 3000+ is available in both 333 MHz and 400 MHz FSB types, and the 3200+ is 400 MHz only.
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Intel Route:
The P4 2.4 'C' or 2.6 'C' (800 MHz effective FSB with Hyperthreading) coupled to an Intel i865 (Springdale) based board like an Asus P4P800 or P4P800 Deluxe would be my picks here. Both of these boards have network onboard. The Deluxe has Firewire and Serial ATA as well.
Unfortunately, you really need paired sticks of DDR400 memory to get best performance here, so reusing your existing memory is out if you want to get top performance.
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The Radeon 9500 Pro is actually just about as fast as the 9600 XT, and actually a bit faster than the 9600 Pro. Since the 9600 XT is only a tiny bit faster than the 9500 Pro, I can't recommend replacing that card with a 9600 XT. You'd really have to make the jump to a 9800 Pro/XT to see anything worthwhile.
As for SSE optimizations in the ATI drivers, both the Athlon XP and P4 can support SSE instructions. It would be SSE2 optimizations for the P4 that the Athlon XP can't make use of. (The Athlon 64 3200+ and Athlon 64 FX Model 51 can, but they are still too expensive.) This isn't really a deal breaker though, as the ATI drivers have optimizations for AMDs 3dnow! in them as well.
If I had to compare relative gaming performance, I'd say the 2500+ Athlon matches up well with the 2.4 'C' P4. The 2800+ and 2.6 'C' are very close as well.