I got my 9800GX2 for 350$ including shipping and taxes about 1 week ago. It performs very well but it suffers from microstuttering, which means that it may lag at 35 fps where another card may not. I think it's great for what I paid for.
In my opinion, you have two options:
1) If you have no intention of using SLI later on, get the 9800GX2, it has outstanding performance for the price. BTW, it isn't as stable as other cards in SLI.
2) If you want to future-proof your rig, go with the 280GTX. It may cost a bit more, but you'll pay it off with TRI-SLI(instead of changing the card every year). Additionally, you won't suffer from micro stuttering.
I'm sure the 4870x2 delivers outstanding performance, but it was over budget for me. It was either the 9800gx2 (with a rebate), or the 4870.
Here's what I mean by stuttering. This video shows the worst possible case. We're comparing a better card, in efficient SLI, to a cheaper card, in unefficient SLI.
Check out the stability issues on the 9800gx2.http://www.youtube.com/watch?v=oYh-tsWfQ3U