According to the system specs your PC comes with 1 gig of RAM, you want to up that, looks like 2 is the max for the system.
I'd get a Radeon 5670 instead of the 240, faster and same power use. Maybe a few bucks more, but worth it, it's right at $70. You can move it to a new system later on and it will still be a decent card to use even with a new CPU, etc...
Although a P4 is pretty slow compared to any current system, a 3.4 gig P4 will be able to play quite a few games still when paired with a decent card. WoW, Half-Life 2, etc... Should even be able to handle newer games at lower settings even. Just not with 1 gig of RAM.