I only have positive things to say about Windows 7. Compared to XP, it does consume a bit more ram. About 1 GB for 32-bit and 2GB for 64-bit for Windows 7 and it's like 512-1GB for XP.
Compared to vista it actually seems to consume less. If you have a 3 or 4 GB system and some extra money, definitely go for Windows 7. It also has a lot of other little features that make it easier and more fun to use as compared to older operating systems.