A GeForce 7800 GTX cannot beat a Radeon X1800 XT. If you want to buy the fastest single card, it's the X1800 XT. Most of the time.

Another reason to avoid dual card configurations is that it doesn't always work properly. In circumstances where there is no supporting "profile" or the image is seriously compromised, you're left with single card operation.

Then there's power and heat problems.

All in all, dual card configurations are inelegant, and not technically admirable