The GTX+ setup will be faster where SLI is supported. I always prefer though single more powerful card, it is a simpler solution. If you already have one of the GTX+ cards then that could be a better option.
Rolli brings up a good question; Do you already own a GTX or are you considering purchasing two? I would not purchase two 9800GTXs, however if you already have one this seems like a good way to maximize performance for not too much money.