Hi J.
I’ve never seen a buffer underrun error message from AE. It echoes back to the early days of trying to write to CDROM – Nothing to do with AE, unless you’re trying to burn to disc without rendering – which I doubt! What message exactly do you get, and what are you doing at the time?
Buffer underrun implies data is not being made available fast enough to your host process – let’s assume video playback system; though since AE does not try to play back video in real time if the data’s not available it’s a surprising problem.
By and large, for system speed, you’d likely want to keep your hard disc(s) defragmented, keep a good amount of spare hard disc space, and if at all possible keep your video and AE project files and temporary files on a drive separate from the system drive …
This sounds to me like an error that you may need to fix, and not one that additional hardware will resolve.