Here's a tip: spend a few hours on "computer row" down in Bellevue. That's NE 20 St. between 140th and 148th. There are 7 or 8 small to medium sized computer stores just on that one strip. The following have nice displays of cases that you can look at:
PC Club (top of the hill at 148th)
Computer Stop (behind the camera store)
CompuCare (bottom of the hill at 140th)
CompuSave ('bout a block up from CompuCare)
Each of these has a larger variety than Best Buy and they cater to people who build their own systems.
You could also check out Fry's Electronics. It's a longer drive (they're down in Renton at the Boeing exit), but they have a good variety.