The reason Dell (and Sony etc.) disable Intel-VT and AMD-V is that they cannot support it. Enabling the feature would mean they would have to provide support on it, which the simply cannot do, due to insufficient knowledge at the supportdesk, mainly.
That is, at least, how Sony formulated it.
I tried prying the reason from Sony support guys and that is the only thing they would give me. I finally was able to patch my BIOS and enable VT myself, though.
As for the rest, stuff like Bluepill are not exactly mainstream. And as far as I know - and I work with virtualization stuff a lot - there is no downside to enabling it. If there is though, I would really like to know about it...
One notable thing that I feel has been left out is that VirtualBox is an entirely different class of virtualization. VirtualBox falls under the category of "workstation virtualization" (also known as Desktop virtualization) which is fine and dandy, but it doesn't really compare to Microsoft's Hyper-V or VMWare's ESXi.
Hyper-V, ESXi, KVM, and Xen are all hypervisors - they are tiny OS's that do one thing: run VMs. You don't really interact with them after setting them up. (You can, and should regularly, but it is not the primary machine you interact with. Just make sure it's updated, etc.)
VirtualBox is much more similar to VMWare Server or Microsoft's Virtual PC. That is not to say you can't do anything within VirtualBox that you can't do in, say, Xen, but it's not the goal.
Given all that, though, I would personally not recommend VirtualBox for a production environment - not a lot of trust there, since you have to run a full OS below it (rather than the slimmed down, small footprint hypervisors mentioned earlier). I do love it, though, for testing a server on my workstation and then deploying it to the ESXi cluster we have once I've ensured it meets all the requirements.
(whew, that was a big post. yay virutalization class in college!)
Best Answer
It does look like old versions of ESXi will work on the DL580 G4, but nothing newer than ESXi 3.5 U5. (direct link to HCL entry) At this point, the 3.5 release is getting pretty long in the tooth. Many of the pieces of functionality are present in both 3.5 and 4.1, but due to the age of your hardware, the platform you are using as a learning environment will slowly get more and more behind what's actually happening in the industry.
So - will it work? Looks like it will, as long as you don't mind running an old version. If it's an option, though, I'd highly recommend possibly picking up a G5 or G6 model, which will be useful to you for a much longer time.