If you're planning on using LOTS of softsynths, it'd probably a necessity. I've got 2gb and I'm running reason with reason pianos, abbey roads, and reason drum kits refills. If I have a patch from each of these running, it usually bogs my system down quite a bit, especially if I'm running rewire to protools at the same time.
Even if you're not running lots of softsynths, you'd probably see a noticeable performance bump.
Also, don't buy ram from apple. You can buy the same ram at bestbuy for way less (not to mention any number of online computer stores for waaaaay less.)