I ended up going for the 7900 GTX yesterday after finding out about the Dell deal, where the X1900s didn't have a clear price advantage anymore. The fact that it was a BFG card, which I've found carry a higher resale value on ebay than other brands, was also a contributing factor.
Unfortunately, I just discovered that the alt-tab BSODs I've been getting in two older games are not related to SLI as I had previously thought, but have something to do with the 7800 cards and/or Nvidia drivers. It's a strange problem, especially considering that my old 6800 GT had no issues with the same drivers. I wish I had known this before I ordered the card; it would have probably swayed me in favor of the X1900XT. I could probably still sell the card on ebay for a nice profit given the $390 price I got it for, although that may not be worth the effort at this point.