Well, considering I've only ever had experience with a stock clocked 9800GX2 my advice might not be of great use. However, what sort of budget are you on? D'you have to have either a 9800GX2 or a 280? Have you looked at either a 1GB 4870 or even a 2GB 4870 X2.
I was massively disappointed with the performance of the GX2 when AA was applied in Crysis and STALKER: Clear Sky - this was at 1680 x 1050, highest in game settings with 2, 4 and 8 times AA applied. On a stock Q6600 with 4GB DDR800 - it was a works rig.
I can only recommend you read these articles...
Bit-Tech Review ATi's 1GB 4870...
... the 2GB 4870X2
These reviews include the GTX 280 and 9800GX2 so you can see where these cards are coming in comparison. The reason to why I suggested the ATi cards is the fact that they're much better value for money (or were - depending on nVIDIA's current pricing).
I hope that helps, if you haven't already made a thread asking for advice, I suggest you do so, we've got some really knowledgeable members here that really know their stuff.
Anyroad, I hope that helps, and welcome to the forums!