The G35 is a 6spd with the premium package, right?

On the positive side, the car is hot right now so you could probably sell it with relatively little fuss. Sticker on a new '04 is like $35k-ish, so maybe you could sell yours for $32-$33k?

I'm assuming you have a loan so whatever you sell it for would go mostly towards paying that off. Assuming you have like I dunno, $8-10k left over after the G35 is paid off then you could go put that down on a TSX and probably have monthly payments around $380. This is all me guesstimating here and I have no idea what the situation with your current loan is.

It's really just a matter of figuring out how much you owe on the loan and how much you can sell the car for, then taking that money and putting it into a car which will yield a lower monthly payment than you have now.