Recommended Space for Vista on Dual Boot System

I'm currently putting the final touches on my new computer and though I was on the fence of going all out Vista i decided to do a dual boot between XP and Vista to be on the safe side (in case one thing doesnt work on Vista i can always use XP). The two OS's will go on a 150GB Raptor Drive. My initial plan was to place XP on the C: drive and dedicate 15GB to it, Vista on a newly partitioned V: drive and dedicate 15GB to it and create a G: drive for the remaining 120GB for programs. I thought this was fine till I read on a site that recommended doing 30GB for Vista because "Microsoft recommend that to take advantage of the 'Premium' features your PC should have around 15GB of free space following the install'. Is this true? I thought 15GB of total space for Vista Ultimate was enough but am i wrong? How much space should i allocate to Vista to give it enough breathing room and not limit space on my drive for programs? 20GB? 30GB? Thx in advance.

I'm looking at the space on my hard drive right now...and to me it looks like you should be able to easily run Vista with 20GB. The problem is that obviously, you wont be able to install anything No doubt you realize that so I will just report, yes, it looks like 20GB is ample

On my PC:
Vista: 192GB
XP: 87GB