From experience, and the fact im running your setup, there are a few things you need to be aware of.
Ive cut my 80GB HDD in half, 35 for XP and 35 for Vista, and that is absolutely fine. you might want to make the vista drive bigger, but not much.
Make sure you install XP FIRST and then install Vista. This way Vista picks up the XP boot option and adds it to the boot menu. If you do it the other way, XP doesnt recognise Vista, and getting it back is a b****!
If you are like me then you will probubly have the first half of your drive with XP, and the Second half Vista. In this setup, while in XP you can set the Vista drive to whatever you want (ive set mine to V: and ignore it), but when you are in Vista, as the boot information is stored on the XP drive, Vista will not let you change the letter. it will probubly get alocated D:, and thats what it will have to stay at.
All my documents are stored on another (200GB) HDD and you just make the XP 'My Documents' folder point to there, and make the Vista shortcuts point to the relivant folders.
There will be no performance hit on any OS, as they just ignore each other.