I want to upgrade from a 8800gt to something faster. I am running a 750w power supply and a q6600 so that shouldn't be a problem. I want it mostly for gaming I use a single tv as my monitor which is currenty running at 1360x768. In the future I plan to upgrade to a single tv 1080p setup. I am trying to decide between http://www.newegg.com/Product/Product.aspx?Item=N82E168... or http://www.newegg.com/Product/Product.aspx?Item=N82E168.... I like both of these cards for there price and dual fan setup in case I do overclock. The 6950 1gb seems to out perform the 560 ti in most cases but is it by enough to justify the extra $15? Even if the cards are overclocked? I plan to order today so any advice is much appreciated.
I would stay away from Diamond if I were you. I have read really bad things about them in the past. While I don't doubt the card itself, which is probably made pretty cheaply to begin with, the company has a reputation for being one of the worst in the customer service side. Trust me, paying an extra $20 to get a card that will overclock better and give you customer support is WELL worth it.
As for your original question, it depends. How much more in the future are you going to be upgrading to a 1080p setup? At the resolutions you are playing at now, both of these cards are WAYY overkill and you will not notice a difference in performance between the two. If you aren't planning on upgrading your resolutions for a while (6m+), I would hold off on buy either card cause there might be newer cards out by then, if not at least the prices will have come down further. If you are going to upgrade your resolution soon, I would go with the 6950 personally, though honestly there isn't much of a difference between the two cards, only a few fps here and there. The 6950 does perform better overall, and I think in the future this gap will only widen.
More importantly, I would upgrade that CPU before upgrading your gpu. Either of these two cards will be bottlenecked serverly because of that cpu. You will probably end up having to upgrade your mobo in the process, but I assure you it wil be worth it. An 8800GT will run anything on high settings while at that 1360x768 resolution. So I would suggest upgrading your CPU/mobo then upgrading to 1080p then upgrading you GPU. This will put upgrading your GPU down the road when prices will be cheaper and you might even get a next generation card.
Brand matters for AMD video cards since their reference cooler still sucks. For nvidia cards the brand usually doesn't matter because the reference coolers are very good and the manufacturer's designs are often even better.
No it's not worth the extra $20. The difference from a reference design and something like a Twin Frozr is 50-100mhz of headroom.
For your resolution both of these cards will be overkill and you should probably go with a GTX 460 1GB for $150 OTD.
I will look into the quality of the diamond. There is an xfx that is around the same price it just has a single fan setup. I should have mentioned that my q6600 is clocked at 3.6ghz. I disagree with your statement about the 8800gt getting good fps at my resolution. I can barely run anything released recently at max settings most go under 30fps. Even recent console ports fable 3, Assassins Creed, and bulletstorm average less than 30fps on maxed settings. I am ok with it being overkill right now since I can reuse it when I upgrade my whole system.
Just to update I ended up going with the galaxy 560 TI. I am very happy with the purchase. With my q6600 at 3.6ghz I am getting a score of p4189 in 3dmark. Which is only around 100 points less then tomshardware reported with a i5-2500k clocked at 4ghz. My fps has improved greatly on every game I have played. Bad Company 2 is getting around 80fps on average compared to 40 with the old card. Assassins creed went from 26fps to around 60fps. Theres more I could list but you get the point.