First off, thanks for the plug drawerfixer - saved me from posting those 4 links myself
I do highly recommend reading them.
Aside for that, I just wanted to say you really don't have much of anything to worry about. Your planned partition layout seems generally good. I would do this:
<li>40 GB NTFS for WinXP
<li>100 GB Fat32 for storage (both OS's will be able to read this, but in order to format 100 GB into Fat32 you'll need to do it during the Mandrake installation and not the Windows one)
<li>512 MB swap (general rule is make it twice the size of your RAM, so 512 is assuming you have 256 RAM. In my case, for example, I have 512 RAM so my swap is about 1 GB)
<li>19.5 GB Ext3 for your / partition (plus or minus some depending on how big your swap ends up being)
So, that's my suggestion. I would also say Mandrake 9.2 is a good distro to start on. Mandrake is what I started out on with Linux (version 8.0 though) and I loved it because I was able to ease into it - testing out the water if you will.
About the MBR, don't worry about it. The general rule of thumb is, install the dumb OS first - i.e. Windows. As was previously mentioned, Windows doesn't play nice and will completely overwrite the MBR when you install it and then you can kiss dual booting good bye if you install it second. So install Windows first and then install Mandrake. Mandrake's installation process, from what I remember, makes it very easy to dual boot and you shouldn't have a problem with it.