/r/buildapc, Please help me build a gaming rig!

Hi,

I used to be really tech savvy - back in the GeForce 2 days. I now am completely lost, and need your help.

Budget: $600-700 Canadian Dollars, including tax. It's not firm, but I can't afford $1000 rigs.

Here are the basic requirements:

1. Upgradeability

2. Graphics - I want to be able to play games on very high settings. I would also like it to be HD capable.

3. Affordability (so, if I need to sacrifice other things in order to get the most upgradeability down the line.. thats ok.) I don't care if it can play blu-ray dvds, I'll probably just mount images, but I want this thing to be my entertainment unit, I have a fairly sweet Tv to hook up to it. Thats about it. Oh, also I will be slowly buying the parts over the next 3 months, ideally cashing in on some post-xmas-deals. I've never bought anything online before, but have opened a paypal account, so... If you guys can point me in the right direction, thanks!

Edit: Grammar and stuff.