well lets see....do you want hard hitting bass? or nice clean sounding bass?
for the deep hitting subs id go with a pair of 12" subs (single in my case) and a good high power (500Watts to each sub) = 500x2 watt amp atleast.
18ga. wire is OUT of the question.... that is so thin you wouldnt get hardly any power from your amp to your subs. go with Atleast 8ga. wiring from the amp to the subs.
your also going to need a decent amp install kit. go with 4ga. wiring...Scosche makes a decent one that can be found at walmart for $25 and it is a 4ga. kit
Sorry cant help you out when it comes to running your PC through your system....ive never tried it.
Heres what ive got at the moment... rockford fosgate deck ($170 Ebay) Kicker600 Watt amp ($280 Ebay) single 12" JL audio sub ($100) some MTX 3way 6x9'* ($100)
Kicker R13 tweeters ($75) and the amp wiring kit by scosche ($25). all speakers soon to be upgraded to kicker'* new 2003 line up when cash comes available :P my system i have now is pretty decent, and it will turn peoples heads when coming down the block...
All of this is currently in a Taurus ive been driving while my car is in the shop getting a motor rebuild