We are well past V6.0, there was 6.5, then 7.0, 7.1, 7.2, 7.3. If you follow the forums, v8.0 is being worked on. There is very little support for v6. i am running 7.3 on an HP Proliant ML110 (yeah, had great fun opening the box and formatting the preinstalled Win2k3 with SME 7.3, then claiming the US$200 rebate from MS for deploying it!!)
This version is very stable, more secure than all previous versions, and supports Vista out of the box without having to patch Vista (well, did for me anyway).
Go for the upgrade...