Bit Tech just published a Christmas 2008 Graphics Performance Roundup on their new Intel Core i7 test systems. You can check it out over here.
Frankly, it's actually hard to separate the two in a single card configuration, because it's ultimately going to come down to which games you play and whether you plan to upgrade to CrossFire or SLI later down the line (on the proviso that you buy a board supporting SLI because not all do). There are strengths to both cards but neither has what I'd call a complete disaster along the way – overall, SLI looks to be the stronger option for the time being.
As for the other cards, the GeForce GTX 280 performs well and is mostly faster than the Radeon HD 4870 1GB, but there are scenarios where it isn't. Frankly, for a card that costs a lot more, we find that hard to swallow. The GeForce GTX 260+, on the other hand, trades blows at a similar price point to the Radeon – it wins some by big margins and loses others by equally large margins but for the most part it is on even footing. We can accept that, but we cannot accept that a card which is over £100 more expensive to fall short of the mark.