If you want to spend the least amount of money and get the most from it. buy an HD 7750. You should be able to run it off that 350W as long as you don't do anything extreme with the CPU overclocking. If you want really good performance, but not exactly best value for money, upgrade the PSU to 500W or so and buy something like a 7850 or 660. At that wattage, you can take some liberties with OCing the CPU as well.
If I were your friend, I would've slapped you across your face and run over the CyberPowerPC system with my truck. Then I'd sell the crushed computer for its value in scrap, build the exact same system myself, and turn a profit, which I'd use to buy you a new video card. My point: Building your own is vastly cheaper than buying one. You can also optimize it to your needs. I would've recommended an i3 over a 4100 in a 350W system for video editing.
As the deed is done now (and I honestly don't think you would make a profit selling the CyberPower PC for scrap, I was just trying to make a point), I particularly recommend these video cards:http://www.amazon.com/HIS-DisplayPort-Express-Graphics-H785QN2G2M/dp/B007HYIDTC/ref=sr_1_4?ie=UTF8&qid=1355719857&sr=8-4&keywords=HD+7850
if you upgrade the PSU
if you don't.
Since you do video editing, I would also recommend upgrading the RAM to at least 8GB and at least 1866MHz. This is a good one:
If you choose to go with the more powerful video card (the 7850), you should upgrade the PSU to this:
I advise you take my advice, also the advice I just gave you in advising you to take my advice. I just typed advise so much it has lost all meaning as a word to me
EDIT: I see your budget is 300-400. I wouldn't recommend going over my recommendations. Past $250 for a video card, you get diminishing returns, and the $200-250 range is where the most value lies. I play very demanding games (BF3, Far Cry3) at THE VERY BEST settings without MSAA at 1080p, and I get 60+ FPS. The most you get out of a $300 over a $200 card is the ability to turn up the AA to 4x. I was raised as a humble gamer, and I didn't experience the magic of AA until a few years ago when my rig could handle it. I still prefer saving money over the ability to turn up anti-aliasing.
Also, I see you're in Canada by the link you linked. Go to Amazon.ca or NCIX.ca, both offer free ground shipping, and Amazon offers free shipping and replacements for RMAs within 30 days, no questions asked.
If you REALLY wanted to max out your $400 budget, I'd recommend:
The video card $270 deal expires today sometime, so you have to act quickly upon it and make your choice now. If you're set on AMD, a 7950 will suffice. The 660Ti I linked just above really does represent the best value though, but I don't have time to persuade an AMD over to Nvidia. Let me just say this, and this is the only reason I stick with Nvidia from now on: Adaptive v-sync. But, if you're an AMD zealot, or you have no use for Nvidia features, I will break my heart by recommending this, but I know I'm no better than the other brand zealots if I don't:
Also, another thing is the massive 384-bit bus of the 7900 series. If you don't have multiple monitors, it's highly inadvisable to buy a 7900 series card. Get a 660Ti or 670 instead. The money saved by shaving off that (possibly) useless 384-bit bus is put towards things that will help you out in single-screen gaming.
And if you do go for the no-holds-barred $300 card, you can run it off the 600W PSU I mentioned above.Edited by 267267123 - 12/16/12 at 9:12pm