You have a TI 4200 64MB card that is actually still a good card to play most all the current high end games. Your computer, has an AGP slot so will take a 128MB or 256MB video card and play games well. You don't need a 256MB card however, (re: what I posted here on 256MB )and I suggest you stay with Nvidia cards. You probably will not be able to take the very high end Nvidia cards like the 6800 and 6600 because of their length and needing to take up room on the next PCI slot.
I am wanting to upgrade from my hardy yet outdated nVidia GeForce Ti 4200 64MB. I was told by a Dell support person that 64MB was the highest I could go due to how the computer is made. So that limits my options significantly. I was wondering if anyone has any advice on this, like what is the best 64MB video card I can get. I play games a lot, thus I am upgrading. Any help would be great.