If you need a quad-core now, the Q6600 is the least expensive one out there. It's not that efficient compared to the new Penryn-family, but it costs just the same as an E6850 and is only 60€ more expensive compared to an E6750. For the same price or approximately €220 to €230 you get four cores that can overclock all the way to 3.4GHz. It's also not an overclocker champion, but we didn't expect it to be.
For gamers and "normal" users, a quad core simply won't provide any advantage compared to any dual-core CPU, but in the near future some games will make use of them; for example, Microsoft Flight Simulator X already does.
We can only hope that quad core will become more important in 2008 and that there will be more games and applications to take advantage of it.
We recommend the G0 stepping over the B3 and you can check prices here.