It would actually be more accurate to say that you can only take advantage of more than 3 GB of ram with a 64 bit operating system, either a Mac OS, XP 64-Bit or Windows Vista. The Windows XP that most folks have only likes 2 GB of ram, max, or 3 GB if you tweak it.
If you’re really spending a big wad of cash and getting a whole new rig for primarily AE I would probably recommend a Mac pro desktop as you can pump that baby full of RAM and put it all to use. Bang-for-buck wise I do not really see the advantages of a Macbook Pro over a PC laptop as far as processing power. People talk about build quality a lot but I’m on my third Dell laptop and several other folks I know have them and bang the heck out of them with no real problems.
I myself am about to do an upgrade on my PC editor. I have an AMD dual-core that has been working fine but it was a bit dated when I built it new. I’m going Intel and will be getting either an E6750 dual-core or Q6600 quad-core, a new motherboard to support it, and some FASTER DDR2 ram chips to go along with it (not everyone who does home builds/upgrades is aware that RAM chips have a speed as well and if they aren’t matched with your mo-bo or you mix sticks together you will hobble the speed of all your ram). I’m going to be looking at a pretty significant performance boost for $350-$450.
Depending on what you’re working with renders are generally slower on a laptop because you’re only using one drive and it’s likely a 5400 RPM drive as opposed to the 7200 RPM drives found standard in most desktops.
Darby is correct that AE doesn’t give much of a hoot about your video card as long as it is OpenGl. It doesn’t help with renders or ram previews just stuff like wireframes and being a little snappier when you make adjustments.
Other ways to kickstart your system are with faster drives or RAID arrays and making sure you actually have a beefy power supply that lets your baby run full-tilt. I cheaped out on a power supply with my first build and it’s been causing me nothing but headaches (in the form of random freezes, BSOD’s, and other general funkiness).
I’ve only been working with AE since September and have absorbed a lot of this stuff fairly recently so I’d double check everything I just told you before spending money…