Q6600 g0 rev ($190) $90
4 Gigs patriot DDR2 ram (cheapest is $90 ish)$75 (assumption without the actual model of Patriot RAM)
XFX GTX 260 core 216 black edition ($210) $140 - $155
320 gig WD HDD 16 megs cache ($45) $35
Asus P5N-E 2x SLI mobo ($100) only a 650i board, $65 is generous
700 watt PSU ($65) maybe, if Corsair/PCP&C/Antec if lower end brand, $40
Blue/Aluminum Case ATX mid ($50) again, maybe if higher quality case, Antec/Cooler Master etc, otherwise $35
DVD RW Drive ($20) $18
Windows 7 home premium (he isn't really buying a copy)
Adjusted prices for what I have seen same or similar parts sold at by others or myself. Some of those parts have been out a few years now, all are used. I bought a g0 quad for 75 bucks, and got another for free, lol. You can get a newer version of that HD for 49 bucks. 650i board is old now, last review on Newegg was from 3.5 years ago. With no quality name listed on the case and PSU, I have to assume they are lower end brands. Like a case that came with a PSU. Same with the RAM, might be a bit more if it is higher up the ladder for Pat RAM.
Total for that rig, $500 to $550 on the outside.