Yes, the number is reasonable.
Please note that we’re not necessarily talking physical RAM here. Just VM (virtual memory).
Given your PC config I wouldn’t really worry (in fact avast does a pretty good job even on 128M machines…).

And about the ashServ.exe high priority - this is by design. The rationale behind this is that since ashServ needs to be activated on almost all file opens (as it scans files on-open) it should complete its job asap. Hence the high priority… (and it seems to be really working, too ;))

Cheers
Vlk