I agree with fanis. (as per above, and far above)
No point upgrading from one AGP platform to another AGP platform just for the video card.
Increase RAM to 1 GB (1024 MB). - The Intel i900 series supports current generation memory, and large amounts of it, use this to your advantage before Socket AM2 is released, as RAM prices are good now, and will rise around July.
Replace HDD with a 80 GB 7,200 rpm model with 8 MB cache (vs that 15 GB low performance HDD you have now). Using either the ATA100/133, or SATA (+ NCQ) interface. Go SATA if you have the plugs on your board and get a 'real professional' to port your OS + apps to the new HDD (unless you are familar with the procedure, or feel like doing a large data backup, re-installing everything including Windows, then restore data job).
The i915 is not that bad with PCIe x16 Graphics it just isn't as good as other chipsets / platforms are.
Consider CPU drop-in upgrade - Depending on your current CPU, etc