I did extensive research on this because i'm also on the market for a 6800GT at reasonnable price. There is only a handful of online shops who ship to Canada, and one of the better ones is Monarch Computers.
However, don't forget customs fees and taxes along with the added delay due to a strike. I did some calculations between canadian prices and us prices taking everything into account and we are talking about 30-40$ difference. I decided to buy in Canada finally for my peace of mind.
If someone has a better answer about that, i'd love to hear this too.
I wouldn't recommend it unless you are getting a extremely significant price difference. When done with duties, brokerage fees, shipping and taxes the final total is sometimes is within 5-10% of what you would end up paying here.
where abouts are you? if in the gta, I may be able to help you find something reasonable as I'm a reseller.
I only know of NCIX.com, I like it because it offers prices in both Canadian and American. The Leadtek A400 GT TDH is looking like a good deal at the Canadian price point. It's $585.34 CAD while it's $446.82 American. Most GT's in Canada are a bit over $600 I'm pretty sure.
Let me suggest a very good online store : http://www.genitechcomputers.com/ which is a gamer oriented store with a good choice of computer parts and nice price. The service is the best in Canada and ResellerRating show it with a 10/10. I bought a BFG GeForce 6800GT OC there and i fucked up on the payment but they are courteous and ready to solve my mistake over the phone.