Despite ATI's efforts to reduce the idle power consumption of the HD 4890, the real world reduction we've observed is not as significant as the expected 30W, indeed it's just 5W. More to the point, though, nVidia's GTX 260 holds a significant lead in this regard. It's a similar, though less dramatic, story when the cards are under load but here the performance advantage of the HD 4890 compensates for the fact it is the most power hungry. All in all, the HD 4890 holds its own with regards to power consumption.
The ATI Radeon HD 4890 actually brings very little in the way of surprises to the table. ATI has gone away and tweaked the design of an existing product, the HD 4870, and optimised it to go a bit faster and consume, relatively speaking, less power. The result is a card that should be the first choice for people looking to spend around £200 to upgrade from either a card in a lower price bracket or from a card that's a year or more old.
If you already own something like an HD 4870 or GTX 260, then the small performance difference may come as a slight disappointment but I don't feel this is something that can be cause for serious complaint. If you've spent £200 on a graphics card and want to get a significant performance boost nine months later then you should expect to be paying a significant amount more. That said, if you can put off your purchase for another four to six months then that £200 will likely get you a more marked performance improvement.
As for the GTX 275, well looking round the web it looks pretty clear that the GTX 275 performs on a par with the HD 4890 and is priced very similarly, so we'll definitely have to withhold our definitive judgement until we've tested it ourselves. For now, though, what we can say is that you're unlikely to be disappointed whichever card you choose.