In short: How do I know if a Dell-server has support for hardware virtualization once Bios is upgraded?

Longer:
I have a whole bunch of old Dell rack-servers that works very well.
Now I need to run Win2008Server & Win7 virtualized, and I want to use Xen. This means I need a server with support for hardware virtualization (hvm).
None of my servers have hvm-support as of today, also none has an option in Bios to enable it.
From what I've found, sometimes only a Bios upgrade is needed to get full support for hvm. But then the question arises: how do I know which server to choose for upgrade?
Of course I could use trial-and-error, but with a dozen servers to choose from that will take a lot of time. Haven't been able to find any info on Dells site, so I thought I'd ask here first.
My servers are:
Poweredge 750, 1425, 1650, 1850, 2650

If none are good, then how do I know what to buy?
I don't need any disks - got a lot of those lying around - and I'm not that rich either, so I'd prefer second-hand. There are lots of used rack-servers on amazon.co.uk but as said, how to know which I can use?
Or maybe I just need a CPU?

From what I remember, Dell 11G hardware supports this out of the box. Not sure about 10G is what it looks like you have here. The best resource for this is going to be the Dell support teams, they have this info and can provide it to you.

I know that a 1950, 2950, etc.. would work as those are all 11G.

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Pretty sure you need to check the cpu for Intel-VT or AMD-V. Dell model numbers will not help you unless you look up each service code or cpu.

If not then you can still use other VM's and maybe run at 70% native speed or so.

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That is the correct answer, however, dell did not enable the option in the BIOS until their release of 11G hardware is what my knowledge is. Maybe shouldve elaborated a bit more.

We get back to real performance per total dollar then. Many of the older systems can't justify their price. A very modern system running much faster and cooler and less energy per/compute would tend to be a better value. Unless they are almost giving them away it would pay to use quality products selected for the intended use. If these Dells can't be proven to work as planned or use too much energy, then I'd say consider a few new ones instead of a bunch of older ones. I have consolidated some older very good servers into a few or single good ones with headroom to spare.

Unfortunately that info is never present when looking up spec's for servers, only CPU name & speed. I've checked new servers too, both Dell & HP, never any info about cpu capabilities!

... and these servers were actually given to me for free. I'm using them for own testing only, but I can't run more than max 3 or the temperature in the closet will melt them down... (Luckily I don't pay any extra for energy.)
By your comments I understand that the chances for these servers to actually have hvm-capability are very small, I'll look around to buy one. (If people could just stick to Linux there would be no problem, I could do everything para-virtualized, but some clients insist in using that strange system called "Windows" believe it or not! )