I am going to list the cards in terms of actual performance.
The best card to buy around $300 is a used after-market R9 290 (1st pick) (many of them on Kijiji). The next best card is a used GTX780 (2nd pick). Below $280, only HD7970 / 7970Ghz editions make sense as everything else is overpriced in the used market in Canada. More on that later.
If we are discussing strictly new, the next best card under $300 is an R9 280X (3rd pick).
NCIX
Newegg
The next best card is the R9 380 4GB (4th pick). Can be easily found for
<$300.
The next best card (5th on our list) is the GTX960 4GB.
Here is how these cards line up at 1080P depending on the review:
http://www.computerbase.de/2015-08/nvidia-geforce-gtx-950-test/3/#abschnitt_tests_in_1920__1080
Some points to keep in mind:
1) DX12 performance - thus far in the 2/2 DX12 benchmarks, GCN beats Maxwell in similar price brackets, except for 980Ti vs. Fury X. For example, R9 285 beats 960 easily, which means the cards I listed above would win by even greater amounts.
There are some caveats to this -- Ashes of Singularity and Fable Legends are still in beta and may not be representative of final DX12 performance, not to mention we would need a lot more DX12 games before drawing wider conclusions. Still, facts are what they are and AMD is leading in DX12 right now in your price range.
2) NV specific features like TXAA and PhysX. If you need those, your only choice is NV.
3) Do NOT get a 2GB card.
There are certain sites that highly favour NV and continue to ignore the 2GB VRAM issue at all costs. The data is there, including many other games that prove that 2GB is insufficient for 1080P gaming, especially in your price range do not make this compromise.
4) There are a few games that HIGHLY favour NV. WoW and Project CARS come to mind. If you heavily play two of those titles, I would recommend a GTX960 4GB, unless you can find a $300 used R9 290. Otherwise, the rest of my recommendations above hold true if you play a wider variety of games.
You should have noticed that R9 290 is in a completely other league compared to R9 380/950/960 and even the R9 280X. This is why I strongly suggest you consider looking into the used market if your budget is $300 CDN.
Another option - R9 280X is faster than the GTX950/960/R9 285/280/380 4GB but it's actually just a slightly higher clocked HD7970/HD7970 1Ghz edition card. If you look at the prices of these cards in Canada, they go dirt cheap for $150-170 --> so nearly
HALF the price of a GTX960 4GB for similar performance. For example, Gigabyte Windforce 7970 1100mhz for
$200. Once strategy could be to grab one of these cards as a stop-gap, save yourself $150+ and upgrade again in 2 years.
On your PSU, as long as it's a reputable brand like Corsair, EVGA, Cooler Master, EVGA, you are good to go with any of the GPUs I listed.
BTW, I strongly recommend that you consider a used R9 290/290X because prices on them are just very good in Canada. Even if you get a reference blower one, it might still be worth it if you game with headphones.
Here I found a good deal for you:
R9 290X
$320 CDN like new with warranty remaining. The best part is this guy is selling the NZXT Kraken G10 + Gelid VRM cooling kit with that card for just $10 more ($330 total). If you can agree to him shipping it to you if you don't live in the GTA, that's a great deal at current prices in Canada.