You sound like you've already made up your mind. But I'll give you some things to think about:
NV has greater support from developers
CrossFire support is somewhat poor outside of benchmark games. Basically if its used for benchmarks, driver support will come out at some point. But if its not a benchmark game, well, nuts. SLI you can force, CF not so much.
COD4 runs fine on any decent GPU, 3D Mark Vantage is irrelevant. HardOCP have a good article here with max playable settings with loads of GPUs:
http://enthusiast.hardocp.com/article.html?art=MTU1OCwxLCxoZW50aHVzaWFzdA==
Basically the 4870X2 is a bit bi-polar, it (like the 4870) tends to offer either extreme performance, above its price. Or alternatively, offers poor performance. If the games you play are the high performance ones - quids in! NV tends to be more consistent, it performs well in everything.
The 280 is up to £100 cheaper than the X2 too, so there's something else to think about.
Personally I like consistent performance, quiet HSF and the ability to uninstall drivers even if I don't have the card in the PC at the time - so I went with NV.
Oh - don't try to 'future-proof' your card purchase - especially with dual-GPU solutions. You're relying on profiles coming out for those games - as well as the ability for your GPU to actually run them. Taking energy costs into account as well - it might be cheaper to buy two mainstream cards over the 3 years - as well as greater performance. In 3 years time, cards for £100 will offer greater performance than your £350 X2 - while using signficantly less power. I'd rather get a £100 4850 now, then next year get another £100 card, then the year after yet another £100 card. I've got good performance - for £50 less + energy savings + money back from selling the cards.
If you're gonna go high end, then it'll be for current games - not future ones.