Personally, I'd go with 2 servers myself, for redundancy. I've put all my eggs in one basket before, only to have a RAID card crash and burn on me. I'm not a fan of downtime, so HA is now a priority.
As such, for SBS 2011, I'd go with a quad core with at least 4gb of ram, more if you can do it. Also, a hardware raid controller with at least 2 15k RPM drives would be highly recommended for all the activity your having on that box.
I'm partial to dell servers, but really, stay away from el cheapo "service provider" boxes and you should be fine.
