Bigger and better is always coming. The new GTX 270 / 290's are rumored to be coming soon, but I haven't heard any hard dates yet.
Anyways, if I was going to spend around $300 on a graphics card today, my choice would be a GTX260 core 216 or a HD4870. If you fold, Nvidia hands down. In fact, even if you don't, the stock clocked GTX260 beats the ATI card in almost every test while the OC versions add some decent FPS on top of stock.
Personally, I'd always chose a single card solution over a dual. That way, you can always add in a card later if you find your performance lacking. Although with a 22" monitor and either of the cards I mentioned, that won't happen for probably a year or two.
That said, I believe that crossfired 4850's will outperform the GTX260, although not by a large margin. And I've heard of lots of people having issues getting their CF systems to run properly.