Hardware Sizing

If I have an Core 2 Quad Q9400 CPU, 8GB, 3x250GB, unlimited 10Mbps bandwidth server- how many Xen VPS instances can I run with a 512MB guarantee of memory? Does the memory get shared in any way or is the answer 16? Second question, will the 10Mbps bandwidth be a restriction? It’s a deal the server company is offering, but I don’t know if that is a problem.

you need to reserve some memory for the base and overhead, so I’d say 12-14 vps’s on 8gb.
Memory does not get shared with Xen

10Mbps seems not much, in a datacenter usually you get a 100Mbps and optional 1000Mbps

In reality, the answer is: 15x 512MB VPS instances on 8GB RAM, since you need to leave some RAM for the host OS to run as well