Back when I was just getting started with virtualization, I ended up the same way - A classic SATA Hard Drive will not provide you with enough IOPS to run more than one VM actively at the same time - the ~100 IO just don't cut it. Furthermore, vCenter is quite an IO intensive little beast - so you will have a huge benefit from installing this VM to the SSD. Better yet, try having all your VMs there in Thin Provisioned format, and keeping only the most idle VMs (eg. domain controller) on your spindled hard-drive.
Good luck!