Here's the BEST advice I can give you right now:
SAVE YOUR MONEY.
You will NEVER recoup modding costs when you sell your car. As far as I can see, this economy is headed straight down the sh!tter faster than a Mexican burrito, and you'll wish to God that you hadn't spent your hard-earned paychecks modding your car. Trust me. Been there, done that.
Dammit, I sound like my father...