Just a few things:
RAM: have both value and performance, in 256-1 gig sizes, with dual channel kits as well
Video: Perhpas stok a few low ends (ATI 9600s perhaps), definatly some 6600GTs, in a few idfferent brands if possible, and a few higher end cards, 6800GT and ATI equivlent. Make it lcear to your customers that you can order a card of you don't have it right there in store
Hard Drives: I think many consumers are wowed by huge HDs. Stock some 120 gig, 160s, and some really big 200-300s
Random stuff: dont forget the cheap small stuff, for convenince. Stock lots of snagless cat5e and cat6 network cable in various lengths (and make sure to have some crossovers). stock some medical grade alchol and some Artic Silver 5 for the builders (should be able to get good prices, as I would think many would pay a decent permeium to have it immediatly. Some canned air would be good too. Assorted cables, Round IDEs, Molex splitter/extenders, fans, go for lighted ones, and the plain but ultra quiet. Have some screen cleaners on hand. Lots of CD-R spindles and cases. Ahhk, I'll just list the rest:
Memory cards/thumb drives
USB cables
perhpas a bin of assorted screws (I had a heck of a time trying to find some self tapping ones for my fan)
some PCI slot coolers
wire wrap
zip ties
Oh, and perhpas you should consider this, especially if your store is in a rual area: get a kick-a** internet connection (preferably cable), and offer it up to customers who want to download stuff, for free (or you could charge if you have a business-class agreement with the ISP). You would probably use it for the business anyway, and it mmight help to inspire some loyalty.
