I'm not really sure what exactly it is you're asking but a good quad core to OC is the Q6600 - but if you want to try out one of the newer 45nm ones you can. Unlocked multipliers really aren't that big of a deal and unless you're going for a very extreme OC then it's also probably not worth your money.
I read somewhere that a Q9650 had a locked multiplier and couldn't be overclocked, wondered if the rest of the Core 2 Quad's (Q9550,Q9450, etc.) could be OC'd
my Q9450 can go from 6 to 8 multiplier. And as far as i know. from what i've done even if the multiplier is locked you can still changed the FSB to overclock the CPU.
you can over clock any of them, all the X series does is give you an extra option.
FSB is the main limiter in the real world, with a quad you may be able to see better results by under clocking
the CPU in favor of FSB. Nehalem, Intels next gen, gets rid of that by replacing FSB with QPI.
QuickPath Interface is much more multi-core friendly.