Looking at the list for maximum $4500 credit...
Toyota Prius
Toyota Camry Hybrid
Honda Civic Hybrid
Lots of hybids but I am not interested in those...
VW TDI cars
Honda Civic GX
Interesting thing about this car is it uses Natural Gas and from what I read... people gets about 38MPG overall... much higher than EPA rated and there is $4000 Tax Credit. Intersting thing also since you are not buying gasoline that has federal and state road tax... this is also another potential big saving.
All other cars... diesels and hybrids cost a big fortune... $40,000+ like the BMW 335D, Mercedes E320 CDI, etc... Biggest kickers are the large car such as Lexus Lh600 or Mercedes S400 Hybrids that cost $100,000+.