i was gonna go the route of sub first etc.
but u'll prob find it better if u get some good 6x9s or 7x10s and put them and a headunit in. u'll get better bass and sound than a stock system. then save for a sub.
if ur not desperate, look around on popular forums (fast fours, hot4s) in the for sale sections. u can snap up some bargains sometimes. i got a nice pair of kenwood 7x10s 220W 3 way speakers for $25!! the guy need money to pay rego so he was desperate.