"First off how many pc can use a single key?" - only ONE
XP Mode is only worth having if you have software/hardware that doesn't run in Win7. Note that XP Mode requires a CPU capable of hardware virtualisation to work (run SecurAble click here to check this from within Windows). An XPMode-like arrangement may be achieved by using free hard drive virtualisation tools, e.g. VirtualBox click here. Compare Win7 versions at Wiki click here.
Well, as the deadline for pre-orders is over (expired couple of days ago), £77.79 will most probably be the best. But if you use a free delivery option you can get it for less click here = £69.75. Free delivery via click here (easier) or click here.
Also have a look at the PCA dedicated Win7 forum section click here. G