this is what I installed:

in order of installation:
1) alpine 9831 HU
2) Inf Reference 6000cs components up front
3) Inf Basslink in stock sub location
(with sound deadening material in door and behind the seats)

i wanted something to improve the quality of the stereo without spending too much money.

any of the upgrades i made individually made such an improvement. after the HU i was like 'wow.' then after the speakers i was like 'damn that sounds good.' then after the basslink, i was like 'holy sh*t!' i spent maybe $700 for this low budget setup and i'm sooo incredibly happy!

i installed everything except i needed some help from bestbuy to mount the basslink. even the guy there said that he wasn't expecting it to sound great, but was really impressed by the sound.

just want to say thanks for everyone posting their opinions and installation how-to's. couldn't have done it without this forum!