well, I dont have a machine that has 512Mb RAM, but I can say this, the "bare" minimum to run Windows 7, along with Vista is 512Mb RAM, but it all comes down to what specific version of the OS you are using.
I do have how ever a computer with 756Mb RAM and installed Windows 7 on it, and it did run slow at first, but after I changed the theme around, and tweaked startup entries, it ran pretty smooth afterwards. And I have Windows 7 dual booting with XP Home Edition, and after all that Windows 7 ran faster than my version of XP.
That test was with Windows 7 Ultimate RC1, dont feel like trying a lower end version.