Specs:
OpenACS version 5.2.3 Disk Storage Needed: 10 gigs
Hub.org has enterprise level VPS hosting running on freeBSD and reasonable rates for any quota overages. http://hub.org/en/services/plans.php
I've been happily participating with customers using Hub premium to enterprise-ha accounts for years. Their uptime stats available here: http://hub.org/en/about/uptimes.php
cheers,
Torben