"n/a" is not an option for monitor resolution. You have a monitor and we need to know what resolution it is. If that resolution is 1600x900 then you'll need at least a 6950 or 560ti to hit ultra (and even then...maybe not) and you're not going to do that on $700 CAD.
***If you can find a GTX 560ti for cheaper than a Radeo 6950 then that's what you should get (they're basically the same card).
Total of $692 USD. You can try to find the best CAD prices - I never know where to look. At this price point, the i3-2100 is your only sandy bridge choice. If you need to downgrade some things to stay under budget then your first choices are the motherboard (micro-ATX or H67 chipset) and the RAM (4GB is an acceptable starting point). Do not downgrade the PSU and I don't know of a cheaper case or a cheaper reasonably-sized hard drive. You should make the graphics card the last thing you downgrade. A Radeon 6870 is still a good choice.
For reference, here is my configuration and it barely manages BF3 on ultra at 1600x900: i5-2500K (stock clocks), HD 6950 2GB (stock clocks), ASUS P8Z68-V Pro, 16GB RAM, etc.