I don't usually test overclocking with video cards because, well, it's typically a real pain in the rear. Video cards crash in some bad ways, and finding the limits of your GPU can be incredibly tedious. However, I decided to give it a try this time around, since AMD is touting the clock speed headroom built into the 4890, and I came away surprised by a couple of things.
First, although it first disappeared a while back, I hadn't realized Nvidia never restored the GPU auto-overclocking function to its downloadable system tools suite. Back in the nTune days, that feature was remarkably good at finding the practical limits for your GPU. Apparently, it has evaporated into the wind, never to return. (At least not in Vista x64, anyway.)
I was even more shocked to find that the overclocking utility built into AMD's Catalyst drivers proved truly useful. My past experiences with this utility were not, shall we say, good. Lots of system lock-ups, very little progress toward higher clock frequencies. But in this case, with the 4890 OC, AMD's utility didn't lock up once and methodically found its way up to a 990MHz core clock with 1190MHz memory, a config that proved wholly stable in subsequent testing.
Since I wasn't willing to endure hours of trial-and-error with the GTX 275, I simply tried setting it to the stock clocks for the GeForce GTX 285: 670MHz core, 1550MHz shaders, and 1300MHz memory. The card was generally OK at those speeds, with no visual anomalies, but it wound up locking up right at the end of our benchmark tests. You'd probably want to back it down a notch for everyday use.