1) Intel
2) No, SLI is limited to nVidia chipsets which IMO are far inferior to Intel chipsets in every respect. Plus, with what's available out there today, I'd recommend an ATI video card anyway, so SLI would be out of the question on two fronts.
3) I see no reason why you could not reuse your 360GB HDD
4) See post #2, specifically, I'd go with an HD4850 in your case.
5) You can keep the hard drive, and possibly the case. What are the specs of that cooler master PSU? It's possible you MAY be able to transfer that to the new build depending on the quality as well as the overall specs of the new system. Your optical drives may be able to get moved over as well.