Yes get that x2 3800, and it will run on that motherboard. just make sure that the cpu says socketAM2 though instead of socket 939 because your mobo is socket AM2 . And by the way the only reaosn he can't activate both cores is because he bought an alienware and they do weird things to their prebuilt pc's. It says right on newegg.com under the specifications that is suports AMD x2.
"CPU Type Athlon 64 X2/Athlon 64 FX/Athlon 64"
New build in progress, but this is what I got for now:
-AMD x2 5200+
-Asus M3A78-EM AM2+
-Buffalo 2x1gb ddr2 800
-Hitachi 250gb Sata HDD
-Antec Minuet Case w/ 350w psu
-Windows 7 Home Premium 64 bit