I think it depends upon the resolution and details that you'd like it to run. I'm running Realflight 4.5 under the 1024*768 resolution, and after some initial screen hiccups things run fine.

You may want to check out Phoenix if you have slow hardware as it is much easier on the machine for similar graphics/framerates. For fancier wind/weather effect I'd recommend Reflex XTR.

Another downside to Realflight 4.5 is you must carry the included interlink usb-radio, so if you're on the road and you want to use your own TX, you'd have to carry two TXs.

I'm running bootcamp on a 2 year old intel imac with 3GB of ram and the basic video card that came with the computer. I have heard that aerofly makes a native mac version but haven't tried it yet...