at stock speeds the GTX275 gives you an average of ~ 1 FPS more in games at high res (1920x1200 w/x4AA) unless you include dead space, then it jumps up to an average of ~ 5 or 6 FPS more (the GT275 just killed the 4890 in dead space scoring more than 30 FPS better for some reason) so if dead space is your game it's a no brainer, also (jmo), when factoring in power consumption and price it's a landslide for the GTX275...Nvidia released the GTX 275 to compete with ATi's upcoming HD 4890. Or ATi released the HD 4890 to compete with Nvidia's upcoming GTX 275, whichever way you like it. They perform around the same, with the GTX 275 perhaps edging it in stock speeds. The HD 4890 overclocks much better though. Some are available with 1GHz core clocks.
Here's a review using the latest drivers.