Windows 7... i havent ran into any issues with win 7 yet, i recommend all my customers that come in with vista, to upgrade. For you guys w/ older hardware, go ahead and upgrade, i havent had any issues yet upgrading an older computer that came with xp, to win 7.
To sum this up in a round about way, win 7 is nothing more then the best parts of xp, and the best parts of vista, joined together, and a couple new features added, and named win 7.
If you have a vista machine, it will run GREAT with win 7, vista needs 1.5 gb of memory to run decent, just by itself. If you want to run other programs, such as IE, MS OFFICE, etc, you need more memory

3 gb of memory is the min i reccomend for vista, then it runs fine. Most manufacturers started doing this about a year after vista came out, so in short, w/ the massive hardware requirements vista needs, it'll in turn make win 7 run that much better, because its light weight, just like xp is.
Bad part about win 7 upgrade from xp, there is some changes, mostly visual changes, but all the same. Things are renamed, and moved around abit, making it a pain to find things like your used to. But if your used to vista, it's not much of a change.
Hope that helps...
-Jer