this is funny because OS X 10.5.6 runs beautifully on 1GB of ram. For reference, the OS X is installed on an Acer Aspire A150 netbook with bottom-of-the-line integrated graphics card, 1GB ram, and a 1.6GHz single core Intel Atom processor. Have I compared it directly to Win7? No. It did, however, have Windows XP installed on it beforehand, and the difference is unbelievable. From video rendering (I assume because of Quartz) to general 'snappiness' of programs (I assume because of the Journaled format.) Even general usability -- the max resolution is 1024x600. On XP the bottoms of the windows would get cut off and it was not uncommon to not be able to click the OK button without using tab and *hoping* that I would hit the right thing. With OS X it was merely a matter of throwing a command in the terminal to change the display scale. At 0.8 everything is beautiful. I bought a Mac two weeks later.
Thought I would contribute my findings. If you don't believe that OS X is capable on a PC, google Hackintosh. You would be surprised at the number of new customers the exploit generates for Apple.