I was wondering say I had two domains, cheapcars.com and cheapcars.cn Does the TLD affect how the domain ranks in search engines?
Yes it does effect the rankings in search engines, though every SE has different methods for rankings but this factor is quite important.
I'd say it's unlikely to have any major effect. If it's a regional domain - ie .cn, and is hosted in that country, ie China, it will affect the regional rankings. However there's no proof that there's any difference between .com, .biz, .org etc in the general rankings.
Yes, pagespank is correct. Domain extension does not has any effect on Search Engine ranking. However, it does affect the results if user is searching regionally.