Yes, my obs machine is a 8 year old core 2 duo. Not sure about the download times. I'd have to check with a Nikon and unfortunately the USB connection on our test Nikon seems to have died a couple months back and I haven't gotten around to checking it. My 6D and 60D both seem to download just fine even on my Core 2 Duo, so I doubt this is related to hardware.
It's unlikely, SGP is not a dedicated DSLR imaging platform, rather it is an automation platform that supports some aspects of controlling a DSLR. There are other dedicated DSLR options out there but they lack the automation of SGP. I guess it comes down to if you want better automation and a single learning curve if/when you get a CCD or better integration with your DSLR.
I will look into the Nikon download issue but it may be a while as we're sans-Nkon at the moment. If you know anyone that wants to buy a D5100 with a non-functional USB port let me know :-/
Thanks,
Jared