I would look at spending a good chunk of my cash on a quality 4 channel amplifier.
Use two channels to amplify you new front component speakers (Mid range and tweeters) and bridge the remaining two channels to create a single channel to power your new sub.
Any rear speakers you choose to run can be powered via your headunit. This will give you a nice, clear, front-biased system.
You can use the amplifer, your headunit or an inducter to filter out everything above 100-ish Hz to ensure your sub is only reproducing the lows.
You can then filter out this low stuff from all your other speakers, thus making sure they're only reproducing stuff they're truly capable of and
operating at maximum efficiency.
If you have more money to spend, go for a dual amp system. Buy a good quality 2 channel amp (Genesis stuff is very nice, good signal to noise ratio) for your front door components and a Class D amp for you sub. These are very efficient and make maximum use of the current available, but the power isn't very 'clean' (ie lower signal to noise ratio) so they're ideally suited to low frequncy stuff, as the dip in quality won't affect sub performance.
Everyone is gonna have different ideas about what sounds good, and what should be done, this is just my view of a good system starter system (first one) and a very nice system with plenty of upgrade paths, as long as the budget allows (second one). Do with my ramblings what you will!
One more thing, don't skimp on your wiring! If the wires are no good, it doesn't matter how good the signal your headunit chucks out, or how nice your amps are, there'll still be a noticable dip in quality.
If you want some help installing it, I'm always free to give a hand! Good luck mate, let us all know how you get on
