Well, what model is your computer and how old is it. That will depend on what kind of graphics card you can install in it. If you just got the computer it most likely has only a PCIe slot. If it is even one year old then it probably has an AGP slot. Also, the lower end Dell computers used to not include even an AGP slot so it depends on what model it is.
My favorite card right around the sub $250 is the Radeon X800XL best performance per price ratio, it is even a favorite over at Cnet. But I will warn you if you don't have at least a 300W power supply in your computer you may not be able to use it. But that's okay, another favorite of mine is the Geforce 6600GT but it only comes in a 128MB version.
But you really need to check your computer, either in the manual or if you have the original specs and see if it has either an AGP 8X/4X slot or PCIe x16 slot and that will determine which version of the card you should get because most video cards now come in both versions.
For benchmarks on the X800XL see Tomshardware, http://www.tomshardware.com/2005/11/28/don/page7.html
and for the 6600GT see, http://www.tomshardware.com/2005/04/04/good_3d_performance_at_a_middling_price/page29.html
for Cnet's review of the X800XL see, http://reviews.cnet.com/ATI_Radeon_X800_XL_PCIE_256MB/4505-8902_7-31320650.html?tag=tpr