upgrade questions for my HP Rig

ok this is what I have.

I have a HP pavlilion a1357c desktop PC

Case HP mid-tower

motherboard : some Asus motherboard mATX

I would like to change this board, not sure what would be the best for my money?

CPU AMD athlon x2 4200+ 939 the socket before AM2. It does not support DDR2 or the new and current socket so that is a reason I want to upgrade.

the questions are:

1. What would be the best mATX motherboard?

2. if my case fits a mATX is it possible that I could get a ATX for SLI?

3. What AMD CPU should I get?

ops, I forgot

my PSU 500w PSU is thermal take

my GPU: BFG 8800GT OC 512 MB