Obviously your dual 7800GTX's will perform better in Doom 3, but the X1900XTX has no problem running 16x12 4x/16x
in doom 3, so it's no slouch by any means. I hardly see that as a place to boast about SLI beyond showing that the SLI setup just doesn't flinch at what D3 can throw at it. Shoot, the two solution are about tied at 10x7, but the SLI setup hardly skips a beat moving up to 16x12, where the top single cards do drop off while still averaging over 60fps.
But something like COD2 your SLI 7800GTX's should make a real useable difference as no single card can crank the res at max detials in that game. In Oblivion, a single X1900XTX does better than dual 7900GT or Dual 7800GTX, but not better than dual 7900GTX. (Of course Crossfire is the way to go for the best Oblivion experience.) Besides that, I can't see too many games where a single X1900XTX is outdoing dual 7800GTX, so I'm not sure what games he was refering with that comment. Of course, I doubt you spent less than $500 on your SLI setup, so single X1900XTX or XT is a better way to go IMO.