Skipping Vista, Windows 7 runs nicely on low spec kit, certainly does most jobs. Most big corporates still run XP, not looking forward to getting the bespoke apps i have to use running on 7!
Windows 7 won't run that well on netbooks, esp by the time the vendor has put their buggy bloat on. I certainly don't think I'll be getting a W7 netbook any time soon.
As to most big corporates, Mr Student, what rubbish. Sky may, but lets face it, they are a bit of an up-and-over garage door outfit 
Most corporates will replace desktops every 4 years, for tax purposes. Most corporates now do not use Software Assurance for Windows, so the licence is for the Windows provided (though occasionally use downgrade rights, but this is more expensive). Therefore, most corporates go to newer Windows, as soon as that newer version is considered stable (usually after testing first SP). Thats why Vista is king in the corporate world currently, although W7 is making headway, despite still being on gold...
I was thinking of a DIY home build rather than out of the box, 7 runs well on my media box (which is about as powerful as Notebook)
I know people at loads of big corporates, they all still run XP. I received a new laptop a few months ago, it has a 7 key on it, but its running XP!
The problem is all the bespoke apps companies use for their individual needs, if those don't work on 7 they won't upgrade. Our team would refuse the upgrade point blank, programs we use don't work on 7 or have bugs and some of the developers/publishers have gone bust so getting updated version will be impossible.
Problem with putting a fresh build on a vendor supplied PC with an OEM licence is getting the media that will work with the OEM key

Of course, there are a million and one ways around that, but absolutely no discussion of that is allowed here, irrespective of if somebody believes they are morally allowed to.
As to XP in corporates, trust me, the vast majority are well into their rollout of non XP machines

Also, MS has pulled the downgrade licences to XP, so no vendor can provide it. As virtually every company out their has used the OEM licence from the vendor for the past 10yrs, and OEM licences aren't transferrable, that adds to the cost considerably (an XP Pro is around £250, an OEM is around £130 (but has other issues surrounding having to buy hardware to be able to legally use it)).