What exacty do you need? HU, front, rear, and sub with amp? If you shop smartly,
EBAY, you can get a good system for 600.
HU-I like Kenwood. They look cool, and have good features. Plus, you can later add Sirius (better than XM) and the new Music Keg (Even cooler, imagin, 2500 songs. Too bad it cost $650ish) kdc-519, face flips 180 degrees so it doesnt look like you have a cd player.
Front-Infinity. They come with an outboard crossover so you dont have to buy a seperate one. Sound good, and can take a lot of power. (I fed my 6x9s 400 watts each for a while and they took it.)
Rear-Depending on your personal preferance, you dont really need to upgrade your rear speakers if your on a tight budget. However, if you want to you dont really need to go high, maybe Kenwood (not excelon), jbl, or pioneer.(You wont really hear them that much so why spend a lot of money on them.)
Sub-So many choices........I think its the hardest choice.
Infinity Kappa-Great SQ. Audiobahn, alpine, mtx, rf, polk, all make good ones. Id stay away from sony.
Amp-Depends on what sub you get, it needs to be matched with your sub for best result. If you have a little cash left over, get an amp for your highs, Id reccomend you wire your fronts and rears for 2 omhs and then get a 100x2 amp.