The I5 750 is a true quad core, the I5 650 is a dual core with hyperthreading. The 750 is much better for the money.
The I3 is the best bang for the buck, and seems perfect for what you want to do. Don't worry about turbo boost, it's overrated.
BTW, you do know that the I3/I5's are socket 1156 and the E8500/Q8400 are socket 775 right? Do you already have a motherboard, or not?
See, I would recommend a P55 board, but you never know if the Bios is going to be up to date enough to accept an I3 cpu. All H55 boards do though, so I'd probably recommend an H55. plus, H55 gives you onboard video thats not too bad, but sounds like you'll be needing a video card anyway.