you are really going to be pushing it with only $500 for splits, 4 chan amp and a sub
also remember you need a box for the sub. have you factored this into your budget? DIY is certainly the best and cheapest way to go as far as sub boxes are concerned.
for a nice sounding set of splits on the cheap I would recomend Digital Designs, you can get their DD C6.5 splits for $172 from
Digital Designs Audio Direct - Get a bangin system in your ride!
for an amp you are really going to want something aropund 4x100WRMS as something with only 40WRMS per channel is only going to give you about 160W to run the sub where as 100WRMS per channels will give you around 400W for the sub which gives you a lot more options.
could go something like this
Ryda Car Audio - Clarion Car Audio APX4360 4 Channel Amplifier($329.95)
another option would be jaycar
Response Car Amp 4x100WRMS - Jaycar Electronics
then I would go for Pioneer TS-W307D2 for the sub, should be able to get one for around $100
whatever sub you get make sure it is either dual 2ohm coils or single 4ohm coil so you can wire it up for 4 ohms
sure this setup would slightly go over your limit (about $600 not including wiring or box) but it is probably about the bare minimum I would go and should sound pretty good.