Dear the OP.
You seem to think that unlocking a Pii 550/555 to a Quad core is a done deal, please understand that it isn't at all.
It is true that
some Phenom X2s are just fully functioning X4s with a pair of cores locked to make up X2 numbers. However, this is not always the case. The reason these X2s are made in this way is so that X4s with core defects on one or two cores can still be sold (albeit as an X2). Trying to unlock such an X2 results in failure and you will always be stuck with a dual core.
Also, considering AMD make a lot more money on each quad core than a dual core - you can pretty much guarantee that a working X4 being "demoted" to dual core duties will be one of the lower binned chips (ie. in initial testing these chips respond less well to higher clockspeeds and generally won't overclock as well as their higher binned brethren). Therefore, if you do unlock a dual to a quad - it will not perform as well as an off-the-shelf quad when overclocking.
From one of your earlier posts you indicated that you not only play games, but also use autocad and other apps. Therefore I would think that a quad core would be of use to you. Which one depends on budget, but the X4 Athlon IIs, X4 Phenom IIs, i5 750 and i7 930 are all certainly the ones to look at.
Here is a nice in depth review that shows these cpus compared in several tests.