Hi!
I don't have much time to post as my 5 yr old just finished his 2nd breakfast and wants me to play now (he woke me up at 4:40am today).
Anyway, I can SO MUCH appreciate what you're saying about helping friends out! My God, it's like we're living parallel lives in different locations as far as computers are concerned.
I've been through your exact situation before and all I can say is be firm early...I was "Mr. Nice Guy" for too long, putting up with more BS than I should have. Simply tell your buddy to either let you work your magic (I'm with you about the vendor issue as well) or ask the other guys at work to build his system. Don't even try to explain the AMD/Intel issue, I've tried that before...all I heard was, "Waah waah waah, why didn't you get me a Pentium 2 gig instead of this AMD 1.7?"
*sigh*
I feel your pain my friend, that's all I can say. I even had one guy ask me to use my own credit card to buy all of his stuff and he would just pay me cash after I ordered it. Riiiiiiiiiiiiiiiiiight.