As far as I know, 4.1 fixes the speed issue, (especially in the iPhone 3g), so my guess is that it will help your 2g. And 4.1 has been jailbroken, actually it was jailbroken a few hours after 4.1 release (said to be really hard for apple to fix up, being a bootrom JB or something), but the jailbreak itself has not been released. Just don't update yet, wait till JB has been released
Forgive me if anything is wrong, but thats off the top of my head