Cleeve has the right of it. It's time to move up.
I had a somewhat similar set up.... a P4 2.53 GHZ system w/2gigs of PC3200 ddr on a Soyo P4x400 board. Original vid card was 9800pro.....great card in it's time.
When I updgraded to a 7800GS graphics card, I found it was considerably better - but not to the tune of what I'd hoped for after studying a lot of benchies. I put a x1950 pro in the same setup to find that performance had only marginally improved. Major bottlenecking seemed to be the problem. (cpu, bus, etc.)
Swapped out the motherboard and CPU and almost doubled my performance from the x1950pro.
It's worth the time and money just to upgrade the platform.