Try to find someone to buy your old machine and build a new one.
If your RAM is DDR400, your system hard drive (where windows is installed) is 7200RPM ATA100 or ATA133 then it would be worth it to upgrade your motherboard and CPU.
Keep in mind that if you would like to keep your current video card, you will have to buy a board that supports AGP, not a PCI-Express board. Otherwise, you will have to buy a new video card too.
If you upgrade your motherboard and CPU while leaving an older hard drive, slower RAM, etc. in the machine, you will be hampering the performance.
Also, have you formatted your drive and reinstalled lately? That may give you back alot of that performance that seems to be missing without any hardware upgrade required.