Is vista ultimate lighter than home basic?

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Hi guys, I had XP in my system previously. One month ago I tried to install vista home basic and I found that it is too heavy for my system(amd duo processor 2.7 GHz each, 1 GB ram, 128 MB internal graphics memory). So I switched back to XP. Then yesterday I got Vista ultimate and I installed it on my system. Surprisingly it works perfectly. I could even enable all advanced features like aero and flip 3D. It only consumes about half my ram. How is this possible?