Ideally, you would want your page file on a separate and fast hard drive as Indy says. In the end, I don't know. In your situation, it may seem hugely important, but if you are having to swap memory between physical memory and virtual memory, you are already degrading performance.
Why can't you set a max on AC? It isn't going to be SBS. It would be the username and password on the AC instance. That shouldn't be hard.
With a mirror your write times should be slower, but your read times should be as fast as one drive.
We can easily get you down to say 800MBs on SQL AC.
All in all, though, with 28 users, not sure why you're not going with 64-bit SBS 2008 Premium with SQL Server Standard 2008. Then, RAM won't be as issue.
