Yeah, I just boosted my 2 year old powerbook from 512M to 1.25G. If you have an intel Mac, you will be able to run Windows natively on it through boot camp. I've heard that there's no performance hit, in that it's not a Virtual Machine, but that Windows runs natively on the intel macs with a few driver changes (
PC's use BIOS from the 80's still, Mac's have something a tad newer).
Best thing to do, when you buy a new comp, is to go for one of the dual-core macs. The iMac 17" has got pretty good bang for the buck, and the 21" is great as well. I guarantee that once you make the switch, you'll never want to go back to Windows, but you'll still have the boot camp option for some apps that are Windows-only that you can't live without.
X-Plane, as previously stated, has vastly superior flight dynamics and scenery (something like 60 gigs for the whole world). MSFS has better looks. I'd personally prefer the addons aircraft of MSFS. After flying those excellent pay-ware models, it's hard to go back to the regular cockpits.