Welcome to TBCS!
Ok, a few things here.
First off, what are you planning to do with it? Knowing this will help in guiding any recomendations as to what you should/should not buy.
@building it yourself, definitely. IMO it's always better to build your own than buy something pre-made. Completely aside from the experience gained, it gives you a much better understanding of how your computer works, and gives you a stronger sense of accomplishment and ownership.
@buying parts elsewhere,
www.newegg.ca is a great source of parts. IDK about the Canadian portal, but generally the US has prices significantly lower than any brick&mortar stores I've seen.
@monitor, I'm not really sure what you mean by 'huge ones that can be passed as a computer'. Again it comes down to what you want to do with the system. For some things a large monitor will benefit you greatly. I think you might be thinking of CRT monitors in terms of their size; large LCD monitors take up very little desk space, and are truly worth every penny IMO (well, usually..depending on the manufacturer).
@getting Vista for free .. If I'm reading your meaning correctly, this is not something that we discuss here. Are you a student? There was a thread here a week or so ago about an open MSDNAA program that you could get a 1-year subscription for $20. ..actually, I'm not sure if that would work in Canada, come to think of it... Alternatively, you should look around for OEM copies. They'll be much cheaper than the retail copies.
If you decide to build yourself and you have any questions or problems that you run into, most of the people on here will be more than happy to lend a helping hand.
Good luck.