If you're looking to build a gaming PC right now, I'd definitely go with an Athlon 64. You're no more likely to have a problem building one of those as you will building an Intel based system. And the Athlon 64s are quicker at games in the same price range. Athlon 64s are very quick at games and cool whereas Intel's Prescotts (the current Pentium 4s) are pretty quick at games and very hot.
If he's on a tight budget then you might also consider a non-sli motherboard. Only truly hardcore gamers actually run two video cards at the same time. The non-sli motherboards are cheaper. I just built a machine for my sister-in-law using an
ASUS A8N5X with a dual core Athlon 64 3800+. The dual core CPUs don't help with gaming at the moment (she wanted a dual for running engineering programs) so I'd also go with an Athlon 64 3500+ (2.2 GHz) or he could splurge a bit and get a 3800+ (2.4GHz).