To the OP, What motherboard and power supply do you have? Buying a big graphics card will yield you little performance gains if you have an OEM motherboard with PCIe 1.0 and a 400w dell power supply.
Yeah, but if you want to know: The Motherboard, The PSU
A 9800GT is better than a 3870 by a decent margin, a 4850 would be around the same price point and comparable to a 9800GTX/GTS 250
however a 4850 is around the same mark as a gtx260, but alot cheaper