If your going to spend enought money for 2 GPUs then definatly go SLI. Oblivion is only one of a few games that Crossfire can stand next to SLI in performance. But SLI and Xfire are a bunch of crap to make you spend more money. I would go with this X1900 XT:
Try to get it before the 30th of this month so you can get the rebate. The ATI XT costs $473 dollars (with the 30 dollars off). Sapphire is a good company from my experience and people I know. It also comes with two Sapphire Select codes where you can pick two games out of four or something like that.
I would definatlly get the DFI Lanparty.