I ran Vista with 1GB of ram with onboard Video and it works fine for "normal" use I even ripped a bunch of dvd's and did some photo editing and such.. no problems.
That's cutting it close on Vista. I gave 1gig a try and it did ok, but it didn't really start 'flying' until I upped it to 2gig. Contrary to what many have said, it's a very solid running OS. The only thing that would deter me from using it as my primary OS would be the DRM crap.
Forget the 512, it still puts you below the minimum with 128 MB being shared. But that's not the point. In the PC world it is easy for the unknowing to buy a computer that won't even properly run the OS. You go buy that shiny new computer, get it home, and it won't even run the OS as advertised. That is pathetic.
Even the bottom-end Mac can run OS X properly (although it can't run Vista Premium). Some high-end software can't use certain high-end hardware acceleration features with the lower Macs, but the user will only notice a speed difference, not a functional one.