If you're using Xen, you'll need at least 2-4 gigs of RAM free for the Xenserver instance itself. I would recommend you also have additional RAM available, in case customers want to upgrade. Please note that Xen's RAM configuration is locked in, once you allocate it (meaning it's not swappable or burstable), so keep that in mind, please.
Xen and SolusVM both work very well with a mixture of Windows and Linux running on the same system, though Xen handles it a little nicer. Just make sure you have bootable ISOs for each of the OS templates you want to use.
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