1. MSI boards have a pretty good reputation for being solid contenders. Im partial to ASUS myself but thats just a personal preference. My only concern is that you picked an AMD board which while AMD is a great processor, (all my machines are currently AMD) its not the fastest processor out there right now, the conroe core duo 2's are. Consider looking at the Asus P5N32-SLI SE Deluxe Socket T (LGA 775) NVIDIA nForce4 SLI X16 ATX Intel Motherboard if you go towards a conroe proc. Its expensive but has a ton of features.
2. Would seriously start with a single 7900 right now and see what it can do before you go blowing $500 on an SLI system for two reasons. First some games like WoW dont even support SLI so you have the same performance regardless. Second around mid next year we'll be well into DX10 land and you'll be buying new cards to support those features. Unless you will be playing a game that hugely benefits from SLI (like first person shooters) I'd pass on it for now.
3. Just go with 2 gigs for now. The benefit of going from 2 to 3 is no where near as big as 1 to 2.
4. If you have the raptors already sounds good. Overkill but sounds good.
5. Would switch over to a Intel E6700 conroe core duo 2, its the best bang for the buck out there right now.