"The trick to making maximum use of the RAM in your computer with After Effects is to set the Memory & Multiprocessing preferences, including Render Multiple Frames Simultaneously."Paying $300 just for a 64-bit OS doesn't make that much sense, but investing in a new machine might, even if CS5 is nowhere in sight. A Gateway quad-core with Vista 64-bit, 8 GB RAM, an nVideo GT120 with 1GB, and an HDTV tuner with Media Center DVR is less than $800 at stores (recently $900 with 22" screen). That's half the price for just the same CPU performance as a new iMac according to GeekBench scores. Those prices should come down further as the Intel Core i7 and DDR3 memory take over on both platforms, along with the release of both Apple Snow Leopard and Windows 7 this year. In any case newer hardware and software platforms should be reasonably stable by the time CS5 ships.
More info on 64-bit performance and Production Premium can be found in previous posts.
Also, the SF Cutters' 9th Anniversary meeting March 17th is featuring a talk by Adobe Systems' Simon Hayhurst & Karl Soule on "64-bit optimization in Adobe CS4."