Marc Grahamsworth, Dell 2400's are not that old.

These computers are really scaled down 4700's. They are build without a video card slot on purpose. But you are right they are not built for real gaming. They are
Business Machines. (Dell Optipex systems are the same computer) But this guy just wants to beef his 2400 up so to add a PCI video card is the next best thing. The Nvidia 5700 is a pretty good card. Even the 5500 is better then on-board graphics. Just make sure you have at least 512 megs of ram with or without a added video card. Windows XP will use up almost all of a 256 meg chip just to run XP.
After all that, I would not put any more money into a 2400 system. People tend to buy these little boxes thinking they are getting a great deal. If they would only check out the Dell 4700's and watch they can get a deal almost as cheap as a 2400. You can buy a Dell 4700 without any video card. It will come with its on-board active. You can purchase your own video card afterwords and turn off the on-board graphics in the BIOs when you need to. You can do the same for ram too, just be aware that
any old stick of ram will not work in a Dell. Don't think you can go to Best Buy and pick up extra ram and expect it to work...it will not. Dell Computers are FUSSY about that.
Now if anyone really wants to know about how to tweak their Dell systems they should go to the Dell Community Forums at Dell's website. There are people there that have answered these questions a zillion times. This is how I learned how to tweak my Dell box. Be forwarned though. You think people here get upset if you do not use the SEARCH FUNCTION...they are real anal about it there at Dell Community Forums.
EDIT: Also the Mods at Dell are ALLOT less forgiving then any of the Mods here. I had a good friend there that was very well educated on all systems. He got so tired of reading the same posts about Dell 2XXX systems that he wrote funny sarcastic answers after while. He got baned for life from Dell for answering to a person that lacked understanding anything that was told to him in a long thread on his 2400. The OP just could not understand why his Dell 2400 would not work well for gaming. So my friend said: "
You bought a business machine ya DORK..." Ka-boom he was baned.