great choice on mobo.... assuming your friend will never
want to go SLI/Xfire. However, if you think he might ever want to add a second vid card you'll have to go with another mobo, like the asus P5Q Pro or GIGABYTE GA-EP45-UD3P and switch to an ATI vid card like a 4870. I mention this only because I just built a system for a friend who at first said he only wanted 1 video card and would never want to go 2. Well, then after a short time, he wanted to add one more video card but had to buy two ATI's cause his board did not support SLI and he got an Nvidia.
GTX260 is a great card. I had that exact same model before I did trade up. That card is easy to OC to superclock speeds.
q9400 is $180 at microcenter.com - go with that as philbar71 suggested. Microcenter usually has better pricing on CPU's. I think their Q8400 is around $150
Micro Center - Boxed Core 2 Quad Processor Q9400
power supply... i would say spend some extra and get a slightly more powerful unit. I like over planning on the power suppply. That way you might be able to re-use it on the next upgrade. You can get the 610 watt pc power & cooling at newegg for the same price as the 500 watt after rebate. But if you can, go even higher to 750watt.... might not need it but it never hurts.
RAM.. like Druid suggested, I'd go 4gb (2x2gb) from any name brand (OCZ, Corsair, Mushkin, etc). Dont worry too much about timings. You'll spend about $15 more, (after rebates) but it's worth it.