Well something I noticed (And one site is claiming because of DX10.1 support specifically) is that the Radeons take a very minimal hit due to AA use. They simply scale better with AA than NVIDIA cards now, and at least part of this is the sheer memory bandwidth GDDR5 is giving. This is obviously opposed to their last 2 generations where AA hurt them.
The GTX 280 is unquestionably the best single card. But before the price cuts... it was by far the worst to buy!
For less than a single GTX 280 you could buy two 4870's or even 4850's that lay waste to it.
Now that we have a $150 price cut things are more questionable... but still, two HD 48
50s cost $100 less, and gives better performance than a GTX 280.
As much as I would prefer to keep a single GPU and avoid any possible Xfire of SLI issues, there hasn't been a better time than ever to try multiple GPUs. And right now, HD 4850 Crossfire looks to give the absolute best price/performance, and best performance short of two 4870's for xHD gaming.
What is even funnier, GTX 280 SLI scaling is just a joke. HD 4870 Xfire still competes well against $1k+ SLI setups.
Edit:
Okay, let me put it this way. Look at FiringSquad's review... the HD 48
50 Crossfire beat the GX2 and GTX 280 in
every single test at 1920 or 2560 resolution. The only thing better, is two 4870's....