I would agree that the single biggest increase can be had from purchasing a new Graphics Card.
I would stick with the ATI Radeon 9600 series, or maybe even pro, because the price/performance ratio I think is better than the ATI Radeon 9800.
If you wanted to upgrate the whole lot!
Motherboard A7N8X-Deluxe 149
Athlon XP 2600 119
RAM 2 Sticks 256mb 120
Radeon 9600 128mb RAM 130
These prices should all be a bit on the high side. (Depending on where you shop.)
You could, in fact, even use your current slower memory and see a substantial performance boost! Although the faster memory would be advisable.
Often I just buy PC2700 memory, as I don't think there is much of a difference between that and the PC3200. That would save some too.
That's my 2 cents.....