Mmm, I just handled an MSI board with an nForce3 chipset a few days ago, one of the nicest boards I've ever used, I don't think it was SLI though.
You should start with 512 if you want to save some money, if you seem to be encountering some problems with performance you can easily add another stick later.
edit - typo, meant nForce3
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w