Depends on which one and how many objects/parts are included, their visibility, their influence, etc...
- Body shell : I would go 4k, 2k minimum for a race-car with livery. Can get down to 1K if the body isn't particularly curved and the paint is plain colors.
- Interior (main part) : same but pretty much opposite : 4k, 2k minimum for a customer car, especially with lots of "luxurious" details, can get down to 1K in a race car mainly full of rollcage and cables.
- Others parts if needed : not more than 1k, 512 is good reference for undershell/hidden parts, doesn't need much more.
Does it affect the result ? Oh yeah, it does. And also remember, you have ONE enemy here : ... COMPRESSION...