Secondly INTEL DOES NOT MAKE VGA CARD FOR PCIe they only have the integrated video in the motherboard which by no means you can play games with, they are only the pioneers in PCI express motherboard technology and most mainstream computers (dell, SONY, HP) usually come with ATI x300se cards in them.
What you say about nvidia being better because of SLI would be the common theory but if you take a look at the article that i linked above or below it has the performance of all the video cards including the ULTRA SLI configuration and according to the testers/benchmarkers in there they said that in some cases the ultra SLI configuration gets beats by the ATI x850 PE. Even though the ULTRA SLI configuration works really good in some games it is highly dependent on the forceware drivers and in some cases if the drivers for the game are not optimised it will give you only a minor gain over one card or sometimes no gain in performance over one ultra card at all. 6600 GT SLI gives you in most cases the same performance as one 6800 GT.
PLus the HP desktop HAPLESS MINDSET has more than likely includes a stock intel motherboard with only one PCI express slot so considering that he will have to shell out $150-200 bucks on SLI motherboard plus pay for two video cards which at the least will cost him $400 ($199 for 6600GT) only for a gain that he may or may not get is not to smart. THis is the article i am referring to
http://www.xbitlabs.com/articles/video/display/2004-27gpu2_47.html