Whats the best, and cheapest, site with the widest selection of cds?

Just gonna suggest Amazon.

Importcds.com has a wide selection, decent prices. Discogs.com you can find just about anything, but prices aren't always very good. Relapse/Hell's Headbangers have a good selection, prices aren't so great. Dark Descent has a good selection of death metal and very good prices, so if you are looking for that genre, I'd check that place first.

abbyss records is pretty good for hard to find stuff, amazon of course, cd universe is a little broader than amazon, interpunk.com has alot of metal suprisingly, just got bathory's nordland 1&2 on b;ue 180 gram vinyl.

I've bought a bunch of stuff from importcd's through Amazon. Great stuff.

It really just depends on what you're looking for. Personally though, my favorite ones to deal with would be Ominous-Domain, Tour de Garde, Forever Plagued, etc.