Dowding, you would certainly see a boost in performance upgrading your graphics card on your current machine. A GeForce 2 Pro runs out of steam around the 700 - 800 MHz level. If you went with a 9700 Pro (or 9500 Pro, which is a good buy if you can still find one) you would be bottlenecked by your CPU, but you should see a significant boost in performance regardless.
If you wanted to do the rest of the system, a 2400+ or 2500+ would be the two best choices as far as bang/buck goes IMO. The 2500+ in particular is interesting, as it has 512kB of L2 cache and a 333 MHz FSB frequency. The 2400+ runs at a slightly higher true clockspeed, but has "only" 256kB of L2 cache and runs at a 266 MHz FSB frequency. An nForce 2 based board is definately the way to go for AMD Athlons. (The A7N8X and A7N8X Deluxe are very nice boards. I personally like the Deluxe model better, as you get a LOT of extra features for a little bit more money.)