Dual boot is easier when Vista is in place first.. Having XP on first, you have to do a Mexican hat dance followed up by a samba to get the OS's recognized.. Vista will recognize XP in the boot loader with a little tweak (which is what the guide is about) but XP has no clue on how to handle Vista and there are many steps to follow to "trick" XP into recognizing Vista..
That being said.. I recommend that you start now to find all the drivers for your system for Vista (64 bit) and make sure your hardware can handle it. Make a drivers disk.. After installing Vista, you will have to load your drivers first and then get Vista SP 1 plus additional updates before attempting to install XP for a dual boot.
but if you have your drivers disk for Vista, the install will go easy.. what takes time is partitioning the HDD and installing updates.. the OS for Vista and XP installs rather quick.
I'm running Vista 64 bit with XP 32 bit dual boot.. works nice and smooth and I have no errors so far.. I just restart system, pick XP and load up.. restart, pick Vista and load up.. no hassles..
Just remember to keep software independent.. IE: if you install OP to Vista, do not try to launch it when in XP.. the regisrty entries are in the Vista load up and not XP and the game will error.. this is true for all software. IF you want it on XP, install when in XP, keep it off the Vista drive completely. Vice Versa for installing for Vista.
Now you can transfer basic files without a problem.. I can even do it with the server kit.. but I have the server kit installed to both OS and I just copy and paste the server kit folder to the other drive, overwriting the old kit install.. but I had to install it to both OS and have it in 2 locations to be able to do that.
but my recommendation is to use Vista first, then install XP.. Microsoft has instructions to Dual Boot with XP pre-installed, but the steps are really a pain to pull off.