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It depends how powerful your video-card is. If it "only just" meets the minimum spec for Aero Glass then disabling Aero will improve graphics performance in other areas. But on systems which easily meet the minimum spec, disabling Aero could actually have a negative effect on performance.
One other point, disabling Aero on a laptop will allow the battery to last longer before it needs recharging.
Processor AMD Athlon 64 Processor 3400+ 4.2
Memory (RAM) 2.00 GB 4.4
Graphics NVIDIA GeForce 8500 GT 4.4
Gaming graphics 1278 MB Total available graphics memory 5.1
Primary hard disk 133GB Free (186GB Total) 5.3
I'm not sure if that is well over, or well under