OMFG I need help with this car situation!?

Alright this is the situation just letting you know its a little long so bare with me. okay I currently own a 2003 Mit. Galant and it has 86,000 miles on it NOW I owe 8,000 on it and i figured if i traded it in id get 4,500 or 5,000 for it so then the remainder would be 3,000 that would go ontop of another car NOW the car i saw was 19,000 and it was a 2007 Infiniti G35 Coupe with Navigaton now the total with taxes and everything would be around 23,000 and i have a warranty that i bought that I NEVER USED ONCE! and i paid 1,900 for it. The place where I was looking for a car said he can most likly get that money back for me so now my question is how much would that payment be for a 23,000 dollar car a 2007 with 26,000 miles on it?? My dad is going to co sign and hes got amazing credit around a 800... I need a round about cost how much it would be thanks !!