The X800XL undoubtedly offers the best gaming "bang for buck" of the choices, but in his case this would be diminished by the fact he'd have to eat a loss on the sale of his used 6600GT. Also, he spent at least $50 more to get a SLI motherboard, and maybe bought a new power supply as well. That money is gone as well if he uses an ATI card on it. If you figure he loses $50 on sale of the card, $50 on the unused motherboard functionality, adding $100 to the cost of the X800XL doesn't seem quite as attractive.
Not to mention he would be going back to the 2002 feature set.
The same loss would apply to the 6800GT, but it being the fastest and most upgradeable solution offsets that as well.
IMO, his best option is to get another 6600GT and use his SLI. Here's why:
1. I don't think he'd notice the gaming difference 99% of the time. A look at the the benchmarks where the the average fps are at or above 50
HERE will show I'm right about that.
2. Doesn't lose money on sale of original and motherboard
3. If he likes to game at 16X12 0X?X, the 6600s may well be faster and smoother than the other two
4. If he's using his computer as a way to watch video at college, the 6600s have the most advanced video feature set.
5. The 2002 feature set of the ATI cards is starting to show it's age: HDR in Far Cry, SM3 in Far Cry, Painkiller, Splinter Cell 3, Soft Stencil Shadows in Chronicles of Riddick, gameplay in general in Doom3 and Doom3 based games, MS standard- all of these tell me I want to have SM3 and 32 bit in 2005
6. As a 6600GT SLI user, I disagree with his "compatibility" issues. If he would download the latest drivers and look in the nVapps.xml file in his system 32, he'd see a pages long list of preset profiles for games that do SLI "out of box" now. He can force SLI on all games with Coolbits. He can point and click to create profiles with utilities. SLI compatibility issues is largely a myth told by people who don't have SLI. While it's true SLI doesn't seem to overly benefit some ancient games, a 6600GT offers smoking play on them anyway, remember on 6600GT is in general as fast(er) than a 9800Pro, last gens best.
If he were a guy upgrading his aging AGP set, and wanted the highest bang for buck ratio and PCIE, I'd say that this week the X800XL on a single slot board offers that.
However, he has 2/3 of a 6600GT SLI rig, and losing money to play with a midrange ATI card doesn't make sense to me. The cost, 2002 features, and performance advantage mainly being at unplayable settings on a couple games would kill that idea for me.