Yeah cpu and ram bottlenecks are inescapable. But most people forget that AXP 2500+ and X800Pro is still faster than A64 4000+ and 9800XT. Remember, for games, your primary concern is image quality otherwise why get a faster graphics card? You could always play with DX6 settings and black and white and get 1000 frames 😉 A faster graphics card will at least allow both of you to increase the resolution and image quality and still maintain playable FPS.
For OP, it's a good upgrade.
for oldman, I'd maybe look at 6800GT and play at 1280x1024 4AA/8AF. Given that you could probably sell 5900xt for $80, a $250 additional to get 6800GT AGP sounds like good money spent. After that you can simply upgrade to A64 and a whole new setup. If you are content to wait, you could always get a new genration of cards and A64 in the summer (but then again waiting waiting waiting....)