I would definitely hold off for a few months on buying a netbook until the manufacturers have had a decent chance to test their hardware with Windows 7 retail.
I have an eeePC 900HA, 1.6Ghz Atom processor, 2 GB DDR2 RAM, 160GB hard drive. I run WIndows XP Home and have no inclination to upgrade to 7.
Why? My netbook was manufactured early 2009 and so ASUS had not had a chance to look carefully at Windows 7 and ensure a base level of performance and compatibility.
More importantly it does everything, including running Visual Studio 2008 I need on XP Home. It might be cool to try Windows 7, but I actually need to get things done with my netbook.
Regardless, read up on this Toms Hardware article if you do decide to get a netbook with windows 7.
The main issue, as with most new OS, is driver related. You can be sure however that Windows 7 will be doing more for you in the way of Security that XP does, so it will be slower by default on the same hardware compared to XP.
Regarding iTunes, I run it "happily" on this same XP netbook with a library of about 4500 songs. Unfortunately the Windows version of iTunes is not the best performer on any hardware (compared to the Mac version). The 2GB of RAM I have is probably more important then anything else for decent performance.