Windows 7 is less resource-hungry than Vista, and from my experience with it I would judge about the same in that area as XP--in all honesty, XP probably edges it out very slightly in some areas. Although 64 bit W7 does bring out a (64 bit-capable) machine's full capabilities in a way 64 bit XP never did.
As I mentioned in my previous post, on my dual boot XP/W7 computer, games run better on the W7 side. They have less graphical lag and higher FPS.
The other reason I'm recommending most people I know to upgrade to Windows 7 from XP (it's a given to do it from the wreck of an OS known as Vista) is that Microsoft has been trying to kill off XP for quite some time now. There will be no more service packs, and my guess is within a year all other updates and support for XP will be terminated. They tried it two years ago, but customer backlash forced them to keep it going. Microsoft wants to be rid of XP ASAP.
And come to think of it, Microsoft is probably going to kill off support for DirectX 9.0c around the same time as they do it for XP, since they have their newest version (11) out and at that point the only two Windows OS's in their minds will be Vista (DX10) and Windows 7 (DX11). The real thing to watch for here is xbox 360's getting an upgrade and start shipping with DX11. Once that happens, DX9.0c is pretty well gone, and game designers will move in the direction of DX11 as well.
Here are some articles with various benchmark testing, etc. Most recent first.
Article 1
Article 2
Article 3
Article 4
Article 5
As far as features in Windows 7, those articles go over them briefly. Aside from that, the
wikipedia article on the subject is a good place to go.
There are a few reasons not to upgrade to W7 from XP.
1) Expense
2) Machine isn't capable of 64 bit or just won't run 64 bit effectively
Seriously, if you're sticking to a 32 bit OS, don't even bother with 32 bit W7, keep XP.
3) No direct upgrade path from XP (forces you to reinstall all your stuff)
4) New interface to learn
5) More devices are (currently) compatible with XP than W7
Personally, I am going to upgrade all of my machines to W7 full version (since I'm using the RTM at the moment) except for one which will continue to dual-boot XP and W7, so I can still run some of my devices that don't have drivers updated to W7 yet. Once those are, it's goodbye XP altogether. When my new MBP arrives, I'm not even planning to put a Windows installation on it, since all my needs from a laptop will be met in OSX. Right down to gaming, since the two games I play most often (EVE and CoH) run on the mac.
Dang...I just wrote a book...