New MoBo but.... what CPU buy????

Looks like you need an Athlon 64 X2 4800+ in a socket 939 if you can find it. Otherwise get a X2 4600+ in a socket 939. Get it soon, the better cpus are going fast.

The socket 939 will limit your options since they have been discontinued and supplies of the 4800 are drying up fast.

Not sure why you would get a motherboard before you've selected a CPU, but I guess it's already done. Kind of like buying a transmission before selecting an engine for a car.

The E6800 if you can afford it or go with the E6600, very affordable. These are the Intel Core 2 Dual. Very energy efficient and kicks the crap out of the 939 AMD brand. My cpu temp is 39C at idle and 44C with BF2 running. Make sure you get the P5B Deluxe from Asus. Get the 2gig of Ram with 800mhz and 2 hard drive and set it up as Raid0. Get the Nvida 7600 video card or you can get two and have dual video. This setup will rock. Very good for extreme gaming. I use the Tsunami Dream case. The best case I have ever use.

But, since you have the 939 board, you will have to settle for 2nd best.