Ok, let me get this straight. Are you:
a) Buying a computer now because you need a new computer NOW?
b) Buying a computer now that you won't need until 18 months from now when Diablo 3 is released?
If the answer is b), then that is VERY dumb. You're throwing money away. Whatever system you buy today will cost about 50% less in 18 months. Seriously, we'd all laugh at you, for your own good. I can't stress enough how dumb that would be.
If the answer is a), you're buying a computer now because you're just fed up with your current slow laptop, then by all means upgrade
Sounds like that is the case, and you want to make sure that this rig will also be able to play Diablo 3 when it comes out. However, you also gave us a $600-700 budget (not a lot). The good news is Diablo 3 and Starcraft aren't going to require the best of the best hardware like Crysis did at it's launch, so that budget should be sufficient.
I have a couple questions for you before I throw together a build:
1) Are you overclocking? (the correct answer is yes
2) Remind me again of the Canadian web sites for computer parts (I know ncix, but aren't there others?)
3) Do you already have a decent operating system, such as XP Professional or Vista Home Premium? Or do you need that to be part of your budget too?
4) What games are you playing NOW? Here's why I ask, if you only play games like CS Source, WoW, etc then you don't need a powerful video card NOW. You can get away with a cheaper one, then upgrade in 1-1.5 years when you need it.