Dimension 2400?
Whatever PCI graphics card you can find for free. At best it has a 2.4 ghz P4 with a 533 mhz FSB.
A PCI Radeon 5450 and 2 gigs of DDR 533 RAM come out to about $160 on NewEgg, and you still wind up with a computer that's mediocre at best for anything more intensive than 480p YouTube and Word.
The best use for that thing is as a secondary Internet PC (after giving it a good nuke and pave), or install FreeNAS and a few extra hard drives and use it as a network file server. It's okay for a beater machine to play with, but for anything more serious than what an Ion/Zacate nettop can do, forget about it.