DDS is stored in the GPU and read in its compressed state, unlike png or jpeg which is uncompressed. TGA (unless your using RLE compression) is already uncompressed but large file size.
And I repeat, the GPU is capable of reading the texture in this state. There is of course some loss of information as a result, but the difference is imperceptible in most circumstances. It is recommended to always use block compression of some kind, except where the texture is to be used as a user-interface element or contains text.
So again, I do all the large UI stuff in DDS (DXT5 no mip maps, 4mb tga to 1mb dds), but the stuff I listed before remains TGA, can still use dds for all but the smalll stuff can be less compressed or not block compressed (DXT1, I think) for quality.
I am still not sure though if the tgas are in the system ram or GPU ram or what difference that makes.
Oh, and never rely on the guy selling you the car about its quality, take it to a mechanic. The seller is a business and will only do the work required to sell the product with the least amount of costs or hours of labor.