depends on which x2 we're talking about...
one thing is, dont get a s939 cpu. amd will no longer be using that platform.
you need to buy an am2 cpu and mobo. if it was me, i would wait until later this year and see what the new amd chips are gonna do. thats just me, but in any event the new cpus will work in an am2 mobo.
to compare to the e6600 at stock speeds you are talking about an x2 5200.
the only better amd would be a x2 6000, and that is about 500+ dollars and not worth it.
x2 5200 costs about $120 less than an e6600. its $200.00
and lemme tell ya, that aint a bad deal AT ALL.
it depends on your budget.
it depends on how much you want to overclock and how important that is to you.
you wont really touch a c2d overclock (performance wise) with a 90nm amd.
but in a stock configuration or with a moderate overclock they will be similar in performance, with the e6600 being a little faster.