A while ago, I switched my Vista laptop from using Aero glass (which it is capable of) to the classic Windows 98 theme that I've been using (with few modifications) for over a decade. That one simple change made the machine quite significantly faster - that much faster that I decided to keep glass off despite it being that much prettier. I think part of the reason is that I tend to keep a lot of windows open at once, many of them maximized. Because the Desktop Window Manager keeps the bitmap for each open window, this uses up quite a lot of memory.
Unfortunately disabling DWM still doesn't give me back the ability to use full-screen DOS applications, which is something I occasionally miss. Still, I can use DOSBox for those tasks now.
Next time I replace my laptop, I'll have to try this experiment again and see if the performance difference has become negligable.