AMD 4800+ Brisbane CPU

Unless you have a seriously low budget get an new AM3 Athlon and a cheap AM3 board or better.

It depends on what features you want on the motherboard. Most Socket AM2 motherboards and just about every Socket AM2+ motherboard will support the 4800+ Brisbane; pick one based on the features it has versus the features you want. I built a friend a computer using a Gigabyte GA-MA78GM-S2HP micro-ATX Socket AM2+ motherboard and it was a pretty decent unit. That motherboard supports the 4800+ Brisbane as well as every other Socket AM2, AM2+, and AM3 CPU, so you could upgrade from the X2 4800+ to one of the latest Phenom IIs if you so desire. That board is probably best if you are looking to make a general-usage desktop or HTPC instead of a workstation or fire-breathing gaming machine since it is a micro-ATX board with one PCIe x16 slot, modest overclocking capabilities, and no ECC RAM support.

The Brisbane is a Socket AM2 processor, not Socket 939. There were a bunch of Athlon 64 X2 4800+ models:

- X2 4800+ Toledo: Socket 939, stepping E6, 2x1 MB L2 cache, 110 W, 2.40 GHz
- X2 4800+ Windsor: Socket AM2, stepping F2, 2x1 MB L2 cache, 89 W, 2.40 GHz
- X2 4800+ Windsor (energy efficient): Socket AM2, stepping F2, 2x1 MB L2 cache, 65 W, 2.40 GHz
- X2 4800+ Brisbane: Socket AM2, steppings G1 & G2, 2x512 KB L2 cache, 65 W, 2.50 GHz

Maybe. Later AM2+ boards will run basically any AM2, AM2+, or AM3 CPU, so he could run his current DDR2 RAM and X2 4800+ and then upgrade to an Athlon II or Phenom II CPU at a later date if he so desires. The only real advantages of Socket AM3 boards have over AM2+ boards would be that DDR3 RAM gives modestly better performance than DDR2 with Athlon II/Phenom II CPUs and the fact that DDR3 is currently cheaper than DDR2 RAM. Although if the OP is sitting on a good amount of DDR2 RAM already, it would certainly be less expensive to just keep the DDR2 he already has. AM2+ boards are still widely available and no more expensive than AM3 boards, so it's not like he would be trying to find older parts that are difficult and expensive to find.