get core 2 duo e6300/6400 theyre brilliant (for now anyway) and even if you dont overclock theyre still brilliant um but then again you could get a 3800x2 am2 its around the same price as the intel e6300 and gives around the same performance (except i think in games) however if you compare athlon 3800x2 overclocked to 3ghz (with water cooling probably to achive this speed) and a e6300 overclocked to 2.5 ghz (depending on motherboard+ram*), the e6300 would win.
so the answer is if you want to save some cash (and stil get decent perfomance) get the athlon, youll still have a good upgrade path to the K8L (AMD's supposed conroe killer). If you want a sytem that will probably be more or less the same performance but uses less power, creates less heat and has the added feature of in the future youll be able to overclock to around 3ghz and probably rival a future cpu, core 2 duo e6300/e6400 is probably the way to go.
*ram for an overclock to 2520mhz (2.52 ghz) with e6300 C2D will need to be able to reach speeds of 360mhz (720mhz ddr). motherboard must also be able to reach 360mhz fsb (1440mhz QDR) dont worry the gigabyte 965p ds3 reachs this shouldnt cost too much.
although if you arent overclokcing and you get c2d "cheap" ram performs well get ddr 533mhz cas 3/4 and youll still get better performance than faster more expensive ram. This is guess means that the cost of an athlonx2 rig is leveled out against the c2d system... theyll be around the same price and the same perfomance ....athlonx2 will use more power and make more heat...it could be cheaper in the long run to buy c2d...