The crucial aspect with Aero is your graphics card. If you have a decent graphics card, the CPU can offload a lot of the Aero processing onto the graphics card, and it probably won't impact on the running of your PC noticeably compared to turning off Aero.
However, an 'average spec' PC.. i.e. with moderately low spec (by today's standards) on-board graphics, will allow Aero to run, but especially if you're doing work that requires a lot of CPU use, you will notice the difference if switching between windows a lot.
And yes, I believe Aero does require a lot of system memory too.