I would say they're about same performance.
The different only on their memory, which is quite useful when you use more than 1 monitor, especially for gaming...
Not specifically tied to 1 or 2 monitors. I have Skyrim modded with HD everything you can think of and it uses pretty close to 2gb limit of my cards (Galaxy 560's)
Edit to Tom Burnqest, I had a nvidia 9600 GT running skyrim on 35 frames on med / high settings and the 450 runs circles around it performance wise.