Make sure you buy *big* disks for your system drive!
I just ran into an interesting thread from one of the SharePoint support forums. As I am sure that you are aware of (sic) SharePoint has a new high severity health rule for SharePoint 2010 that raises an error if your machines does not have 2X the amount of physical memory in free disk space. Yep, that’s right. I will leave it as an exercise for the student to investigate why, but this has a BIG impact on PowerPivot servers.
If you have a large memory machine, for example both DELL and HP now have low-end servers with 256GB of physical memory on them, this means that you must have 512GB of free disk space on them or the SP health rules will start kicking in. While this is not common for SharePoint servers to have this kind of memory (SharePoint typically runs on web server-class machines); it is a good idea for PowerPivot servers to have lots and lots of memory (PowerPivot is an in-memory service that needs database server-class machines). PowerPivot has a HUGE demand for memory.