Upgrading your Graphics card is obvious, but I would also consider an extra gig of RAM.
This is supposedly a loud card, so be pre warned, The X1800 XT comes in both 256MB and 512MB configurations. Which is the GPU's own memory. The card is big with dual DVI and will need you to remove two plates from the tower rather than one, unless you get the X1800 XL which still has twin DVI, but only comes in a 256MB config.
Once you're happy with that and have the money, I'd consider then upgrading your CPU and PSU.