1. Will I need to increase the size of the "/" partition on the HD to allow space for the VB to work/run ? (I was planning on 10 to 15 GB for / partition ) If so how much?
By default, Virtualbox uses /home as the area for the VBox Virtual Machines and Hard Drives, BUT they can be moved if you so wish.
I would suggest increasing it by 15Gb - a VM can normally start off as just a few Gigs but they do "expand" as you install more software and media etc (I normally set my VBox hard drives at approx 8 - 10Gb each)
2. My system sees ~2.9 GB of my 4 GB of memory and I am thinking of allowing 512mb of ram for the VB - should I allow a full 1 GB in in case I later use the VB to test other Linux distributions ? Does this take the ram away from PCLOS when running the VB? I'm not sure what the sweet spot for VB ram is but I don't plan on running the VB very often.
As with most RAM, give it as much as you can afford too - more the better.
You may want to consider installing an .pae
kernel as that will see approx 3.95Gb of your 4Gb, which then allows more for the VBox VM's.
But, as the settings aren't set in stone you can tweak the VB memory settings in 2 seconds. So you can start with 512Mb and add more if necessary.
3. Would the WinXP when running in VB be able to open and read my documents stored on another partition on the HD that was formated using "Ext 4" - that would be something if I don't have to set up a NTSF partition for the WinXP ?
Yup - just need to network it to see your other drive(s) BUT Windows cannot see/read
Ext4 partitions so you would need to add some software which can.
4. Would I be able to replace the WinXP with Win 7 down the road sometime if I ever wanted to use Win 7 ?
Again, Yup - just think of VBox hard drives as you would a "normal" one therefore you can install/reinstall at leisure.
I often - when I have it setup as I like - make a backup copy of my VM hard drive(s) as they can be used again and it saves reinstalling etc. The only real limiting factor is regular hard drive space to store them