Well Core2Duo vs AMD X2 is no battle really.
The Core2Duo kicks the AMDs topnotch processors ass any day.!!!
Dont get a AMD if you want the fastest possible cpu.
AMD Phenom which are 4 cores is also no match compared to the Intels Quad Core which also based on 4 cores.
This is the thing, if you want cheaper machine thats still very strong you got for AMD, something like 6000+ should do the trick.
However if you dont care much about the money and are happy to spend 100 bucks more totally on the mobo and cpu then you obviously should get yourself a Core2Duo, id recommend 8500 which runs at 3.16 and can easily be overclocked up to 4GHZ on stock cooling.
Intel is the way to go at this moment if you got the money and they pretty cheap if you compare what your getting for the money.
I love AMD as well but X2 cores never got the chance to battle the badboi Core2Duo from Intel.
If i was you, id wait until Intel releases their new processor called Corei7( codename Nehalem ).
It should be dropping 31th october, and thats when the prices on Core2Duo gonna drop some.
Your options :
AMD 6000+ 3 GHZ / can be overclocked to 3.2 - 3.3 GHZ.
Core 2 Duo 8500 3.
16 GHZ / can be overclocked to 4 - 4.5 GHZ
^^ 8500 is a beast