[standard Prometheus Response]
Buy them from somewhere trustworthy (the various regional Amazon sites, Play.com, Newegg, etcetera). DO NOT go anywhere near eBay. Always test with
H2testw regardless of where you get them from, because sometimes fakes do get into the legitimate supply-chains.
[/standard Prometheus Response]
That aside, what kind, or speed, or whatever, doesn't *really* matter enormously, unless you're planning on booting an OS from it (in which case, go for a fast one). Personally, I'd go with a known-good brand (myself, I stick with SanDisk and Verbatim). I've noticed that under normal usage, there's little difference between Class 4 and Class 6 for what I happen to do (for me, that means emulators/games, web-browsing, music, and writing, generally).
EDIT: Hah, looks like I got double-ninja'd while I was writing this.

h34r: