Which are still relavent to an SR4 game? I see the Seattle Sourcebook; I've heard great things about it, and it's available there for $9. What else should I get? I've also heard good things about Awakenings; is it very rules-heavy?

Awakenings is superseded by MiTS, mostly.

I would say the fluff books are useful, specially if you intend to play in pre-crash days.

I like the Lone Star Sourcebook, Aztlan, CalFree.
Renraku Arc Shutdown, Bug City and Portfolio of a Dragon can be REALLY fun, but are better explored if you´re playing before 2060.

good luck

Any of the location and background books are still quite useful.

early editions, I'd say the adventures are still good. Some of them are set in certain yeras, but those you can mostly adjust if your group doesn't pay to close attention to the time line. Besides that, I've always been a fan of Prime Runners.

Dunkelzhan's Will is a must have.

Rigger Black Book. Just for the imagery.

I am still trying to find a copy of the Denver boxed set, myself.

Make me an offer.

Buy them all. FASA needs the cash.