i'm thinking you are referring to the different bootrom versions.
there is not an 8GB version of the 3rd generation iPhone.
G1 = original iPhone with metal back
G2 = iPhone 3G
G3 = iPhone 3G[S]
the newer iPhones are shipping w/ a newer bootrom which requires a tethered jailbreak at the moment which is a pain in the rear. everytime your phone dies or shuts off, you have to re-jailbreak. the process is more of a pain. i believe phones after week 40-45 (somewhere in there) have this issue.