I got my pc about a year ago now. Overall it is great and I enjoy the experience but I can see it lacking in performance with the new games here are the specs
Amd 6300 3.7 GHz
8 GB
Gigabyte GA-78LMT-USB3 Micro ATX AM3+ Motherboard
AMD Radeon HD 7770 Core Edition
Antec 450 watt power supply
since if got some more money coming in i can now afford a graphics card upgrade
I would go with a 290 the 270 won't be much if any upgrade from what you have and a 280 is a great card and all, but, 290s are lots faster and cost nearly the same right now. I have a 280x it drinks the power at load its up around 250-300 watts when its overclocked at running full tilt. The 290 is only going to be worse, I would try and get something around 650-800 watts which would run everything just fine.
For a PSUs just be a little careful where you get one; New is better, as they tend to lose efficiency and maximum power delivery over use and time. Not that there is anything wrong with getting a well cared for used one, just check to ensure the thing is good. Also, if you are going with a 290 just check some reviews out to make sure the rail/s can deliver the right amount of power.
Better to buy off ebay from a reputable seller compared to craiglist etc. You'll find heaps of cheap ex-mining cards for sale, some flogged out ones, others will be just fine - just hunt around. A second hand gtx 670/680 is also a good option.