As a general rule, don't upgrade unless you expect 2x performance.
Upgrade your videocard.
Don't get any more system memory on the current motherboard. SDRAM's a thing of the past.
Likewise, popping a new processor in your current rig won't give you a stunning performance boost, even if you also change the videocard.
That's because odds are your motherboard is some sort of KT133 or other SDRAM beast. Not only will you be tied down by the slow ram (and who knows, you might have ATA/66 on the IDE controller)
If you're going to upgrade the CPU, upgrade the motherboard.
And the Tbird boards that are being built today tend to use DDR RAM, so you'd have to buy new ram.
So, if you want to upgrade the CPU side, you're looking at:
Motherboard $75. 512 megs of memory: $100. Athlon 1800 CPU -- $90, or $265.
A video card is cheaper.