Std Speed,
If you are running PC-2700 with a 400Mhz FSB AthlonXP, you are choking its Memory Bandwidth. The first thing I would do, would be to get two matched sticks of at least PC-3200 (2x512Mb). That alone will show you a noticeable boost in performance.
Then I would look closely at the Radeon 9800Pro 128Mb, since the price of the Geforce 6800GT will come down faster than the mercury in a thermometer in the middle of a blizzard.
A good review to look at is:
NVIDIA GeForce 6600 GT: Bringing NV4x to the Masses
Take Care,
Richard