Hi all.
"XP mode" is primarily designed for small to medium size business who can "gradually" replace older XP running hardware with newer hardware and continue to run their existing apps.
This way they don't a) have to replace a lot of PC's at the same time - expensive anyway but even more so when money is tight.
b) can continue to run existing applications until they are ready for an upgrade of their software which they can also do "in stages".
Since XP mode is intended more for businesses the VT feature won't be a problem since all new desktop (and most laptop) machines will have this now anyway.
If you need to run XP in a virtual machine on older hardware you'll have to use VMWARE (player, server or workstation) or VBOX, and you'll only be able to run 32 bit GUEST OS'es.
A lot of games / other stuff needing DIRECT X , Video acceleration. 3D mode won't work (currently) in a Virtual machine.
Some old games might but games are one area where a VM in general won't help you too much.
Any old legacy hardware such as scanners, printers, etc as well as old AUDIO type devices will work fine. Old Legacy "Office type" apps will usually work fine as well - even some old CAD pacakages should work.
As far as any API is concerned - unless you know how to program this stuff it isn't of any use to you DIRECTLY. However an application you are installing might says it needs it in which case install it.
Cheers
jimbo