The only advice I can give to maintain performance, is look at the car model file size.
You can get some nice high detail, low poly models out there.
The best that spring to mind are f5544's Driver San Francisco conversions which are available from the files section under his profile....
http://www.lcpdfr.co...er/15429-f5544/ << Here
basically, the bigger the file size/more complex the model, the more intense it is going to be to run.
And frankly, a lot of car models are really poorly optimised.
Also, I use this...
http://www.gta4-mods...rride-12-f15452
Which gives you a seperate IMG file to put your car mods into so you dont have to mess about with the vanilla vehicles.img and makes switching stuff out, and knowing what you have replaced really easy.