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I have extensive experience in the hosting industry and its been my main business for over 6 years. I've sold, installed and used VPS's from swsoft (vituozzo), ensim, vmware and sphera.
Ensim was the top of this game for a long time but have fallen of drastically of late. The main advantage to their system was that it was a true VPS. There was a management level loaded to the server and then partitions. Any OS could be loaded and managed exactly as a dedicated server.
Vmware is basically the same as ensim. It's completely independent and very stable, like ensim use to be. As with ensim and vmware you could only split up the memory and processor in "real time". There is no way to oversell the server resources. This leaves you with a maximum of maybe 6 to 8 VPS's with 512 mb ram to a box.
The issue you need be aware of with virtuozzo is two fold. The first is that their "independent os" is actually based off an install template. If you modify the os in any way and they apply a patch it will overwrite certain files you customized. Also there are some base root files that are shared among all the users. The other thing, and of far more concern, is that with virtuozzo is that the host can oversell the processor and memory. This is how swsoft tells the host they can have 100's of VPS's on a box. They are banking on no one using their resources all at the same time.
I've had a lot of experience with these and I would recommend finding a host that offers ensims vps (if you can find one anymore) or vmware. Virtuozzo really is nothing more than fancy shared hosting.
Alternatively for the prices of dedicated servers these days you could practically pick one up for the price of a VPS. What would be a better idea is for a few users here to just go in on a dedicated server and host their sites off it. Simple and easy way to go. If anyone needs any help or advice in the hosting realm feel free to hit me up. I'm loving XOOPS so far and this cimmunity has been great so far.