For applications that only take advantage of 1 or 2 cores, the x2 565 3.4GHz will be a lot faster. But if you like to listen to media player, download BT, encode a video and play a game at the same time... the quad core might win out.
Go over to http://www.anandtech.com/
and click BENCH
Then select your current CPU 9950, then the x2 565 and check it out. You can also try other CPUs against the 9950 to see what you think. http://www.anandtech.com/bench/Product/21?vs=204
The x2 565 seems to be a bit snappier at just about all the benchmarks. Also it is a Black Edition and can be easily overclocked with a simple multiplier bump!