These x3 are a shame to multi-core. Dont use them. Try finding an application develloped for triple processors...
Applications are just starting to utilize multicores. Think about how gaming has progressed. Games favored high clocked dual cores over quad cores. Now games are finally starting to take advantage of quadcores. Just because it isn't a quadcore doesn't mean it's useless for multicore. An application that has multiple threads will benefit from multiple cores. I challenge you to find a application that is used by the average user that can utilize a quadcore.
Agree with statement that unlocking is hit or miss thing depending on the CPU you get. But chances nowadays are pretty good.
The X3 720BE will not bottleneck your 9600GT. I have an HD4890 and even at max setting in games, the X3 720BE does not slow this HD4890 down...
If you are on a budget, get the X3 435. It has ability to unlock as well and have higher clockspeed than the X3 720BE at stock so if you do not plan to OC, this will serve you better. While cheaper, it even beats the X3 720BE in some benchmarks. http://www.anandtech.com/bench/Product/83?vs=112
My board is the GA-MA78LM-S2H from Gigabyte and it definitely can unlock the 4th core in a 720 because that's what I did. The GA-MA78LM-S2 will also do it (the difference between the two is the S2H has HDMI output with the onboard video card).
However, it is a crap shoot as to whether you get a 4th core to unlock and be stable. I got lucky. I didn't have to adjust any voltages or anything. I hammered it with Prime95 for about an hour and decided that was enough. I know some say you need to do it for 24 hours, but that's seems excessive. If the thing doesn't hiccup within an hours, I say that's plenty for me to determine it's stable.