You won't notice a difference mate.
To give you a bit more information.
The i7 2600 is plenty capable of running any game today and will be for quite a while. The difference between 1333 and 1600 RAM is rather small, and in games you won't see a difference at all. To put it into perspective, DDR4 running at 3000 makes zip for a difference in gaming too (and that's quad channel taboot).
2 reasons why the 970 is the better choice. It's faster, and consumes WAY less power. To use the 290x you would definitely need to spend almost 100 bucks on a new PSU because it uses upwards of 300w by itself. As indicated here
the 970 uses less than 200w. With your CPU and a GTX 970 combo that FSP (although still cheap) should suffice easily unless it's been abused. Since this seems 100% untouched pre-built, I'd say it's fine.
If you want to upgrade anything else besides the GPU I'd say get yourself an SSD.
"Resolution is just a number." #Ubisoft Origin/Steam
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