True Newbie needs a hand. How to install Vista dual boot mode on an XP machine

Hey all.
Firstly, many apologies if Im in the wrong forum (hopefully Im not) or If the question I have put to you all has already been answered. I will try to explain myself as Best as I can as I really am a novice when it comes to using My PC. I have trawlled across the net looking for answers and thus far, found it very confusing.
I own the PC game called Crysis (which btw is amazing) and I currently run it on my windows XP media centre edition SP2. I own a Nvidia geforce 8800GTX card and have it on max settings for the game. It runs very well most of the time and looks brill but does act up now and again (Im sure you have heard all about the problems lately with Crysis). Now, heres the thing. My card is direx X10 compatible ok. Windows XP does NOT support this latest version as XP is only direx X 9.0c or whatever highest it is (think thats right though). Currently Crysis in XP is only at 9.0c, so it could play even better. Windows Vista is obviously Direx 10 compliant. So, you've already guessed by now what I want to do. I want to be able to play Crysis in windows Vista (I own a copy of it, but its not on my computer) but still use XP as My "main" OS. In other words the primary one being XP, Vista being the second choice.
I have one hard drive which is a 320GB HDD. Windows (C Is on this hard drive so I would assume in order for me to successfully install Vista 32 bit home edition without deleting XP from my system I must partition my HDD. Now, the problem part. How on earth do I do that without buggering up XP?!?
Im totally scared to death of losing all my work, games, saved files, operating files, major programs etc. This is putting me of even attempting an install. Now, before you come and say look on the net for advise, believe me, I've tried already and Im still no more confident than when I started. That is why Im here. Alls I want to be able to do is enjoy Crysis on vista with improved graphics and performance and get the absolute best out of my graphics card and of course, see what it looks like in Direx x 10. I cannot do this in XP.
I want to keep XP though as I can use this for everyday use. I want to simply be able to shutdown (if of course in XP) and boot into Vista to play Crysis, or start from powerup fresh and be able to choose which OS I boot with. Please do not be put off by my waffling (sorry again). I really thankyou much if you have taken the time to read this. All Im asking for is help. Thankyou all so much.

 

1. first you need to make a backup image of your c: partition where you have windows xp. Use true image or ghost or even the free program partition logic which will let you make a copy of your c: partition. This is important in case something goes wrong. Make sure you save the image on an external drive or different hard drive.
2. You need to downsize your c: partition (windows xp) to about 40gb, next create a new NTFS partition for vista about 40gb, and leave the remainder of your hard drive as a data NTFS partition.
3, Next install vista onto the new partition. Below are the directions that I followed and it worked.

http://www.winsupersite.com/showcase/winvista_install_04.asp

http://partitionlogic.org.uk/

 

Thanx for replying.
Ok. So hear me out on this. I need to make a ghost copy/image of my current local disk C drive (XP) over to an "external hard drive" as I only have one internal drive on my PC. (remember an HDD 320GB 1 of).
This is becuase if the installation is a balls up I still have XP on an external drive that I can just put back on my PC. Am I right so far?
Then I need to use a program to partition my drive (windows C) into another one? Can I not just use disk defragmnet on XP? Will that do the trick?
I need to make sure the file type is NTFS not FAT or FAT 32, correct?
Ive heard you need to defrag the drive first then shrink it. Is that true?
After that I simply install Vista on dual boot up and im away.