Windows 7

I am not entirely sure what you mean, Windows 7 is not meant to be a server, think of it as a giant XP patch, that costs several hundred dollars.

You can have a mixed network no problem, although if you are running Active Directory, I believe there is an performance advantage to having all systems Win2k or higher... I think it was some protocol or something, I can't recall, too long ago that I read this crap in Cisco.

I'm not sure I totally understand what you are asking. I will tell you about my results and perhaps that will help you out.
I have a computer which was running XP Pro. It is a Dell XPS gen5 with a Pentium 4 processor and 4 GB of ram. I installed Windows 7 Pro on it (this was a release copy obtained through MSDN Education Alliance about October 1) While I don't have a bunch of facts and figures to back it up, Windows 7 actually made my computer run better. Less wait time during startup and shutdown, applications opened quicker and generally it seems that the whole system is running better.
I had tried to install Windows Vista Home Premium on it and it was a real dog. So if given the choice, my experience is that 7 is a better choice. But the one drawback is that it has to be done as a fresh install as there is no upgrade path from XP to 7.