Overall 7 is more resource-intensive than XP, however it is faster provided you have the hardware to back it up, except you don't, therefore I'd recommend XP.
The 7 kernel is a modified version of the Vista kernel.
The Win7 Kernel, yes it is a modified version of the Vista Kernel. but it is heavily modified.
http://www.techist.com/forums/f9/windows-7-thread-faq-172024/index22.html#post1693063
That post right there. There was already a benchmark done comparing many things from XP/Vista/Win7. I can vouch for the fact that Win7 RTM is much better. Since i am running it myself.