Don't know about different versions, but i could guess that a cheap copy of seven that does not have aero could be faster, even though you could just untick aero in a version that have it so i guess no there would really not be any change no matter what win7 u use.
And yes Win7 takes up more RAM, i would say the double of what Xp uses, on a normal day i use about 1GB of my RAM constantly, i have 2GB and at startup my Win7 takes up 37% of my RAM so it's not as much as Vista but more than XP -- 9xx SOLDIERS SANS FRONTIERS
