I'm guessing you financed your G35 - but lets say you sell it for $30k (not an unreasonable amount) and then buy a new car. It doesn't make a whole lot of sense to jump into anything over $20k. You'd save roughly $200 a month in financing charges, but you'd also lose money on selling your car plus have to pay tax on your new car (which at WA state rats is about $2,000 on a $20k car). That'd basically erase your entire savings for selling your car in the first place!

If you honestly want to save money for a house, etc... buy a Civic, xA, or Corolla. Base models with decent features run $15k, they are cheap to insure, have awesome gas milegae (a big plus now), and have good resale (except xA i think, but who knows).