To the OP, if you care about high ISO shooting, the 5D Mk II (or one of Nikons FF) are going to be much better than anything, including MF.
MF can provide better images, but just like the difference in a 5D Mk II and a Rebel isn't really all that major, MF will be the same way. A 5D Mk II provides much better images when blown up big, and better high ISO. A MF body will provide way better quality at huge sizes (don't shoot a billboard on a Rebel!) But there are major tradeoffs in flexability. I don't know if any of the MF bodies even use USM/HSM/SSM, and I expect the autofocus systems at best are 40D/50D/5D level, I doubt any are near the 7D/1D bodies. Additionally they are bigger, heavier, and much more expensive.
However, if you want the absolute best image quality for controlled studio or landscape situations, esp if you want things blown up, MF is going to be better.
One other thing to consider. I wanted to know more about medium format so I went out and bought an old Mamiya C220. I only paid $150 for it. It is a good way to play with medium format and see what it is about. Of course a proper scan to do the negatives justice will cost about as much as the camera.