What are the rest of your specs? The only problem with your system is the graphics card. If it's new, I wonder why you went with such a slow GPU instead of buying a good one from the start. Anyway, is this a gaming PC? Will you be playing more games? Also tell me how much money you want to spend on a graphics card. Here are some cards that you can choose :
--GTX 750 Ti, the more budget-friendly option. Can play CS:GO on max settings with about 100-200 fps (depends on CPU as well) on 1080p :
(
http://www.amazon.com/gp/product/B00IB9P1KG?keywords=gtx%20750%20ti&qid=1451909320&ref_=sr_1_2&s=pc&sr=1-2 ) You can choose your preferred brand, look around Amazon.
--GTX 960, this is a better card. It plays CS:GO on Max settings at 1080p with about 150-250 fps, again, depending on your CPU as well.It will probably last you a lot longer for gaming than the other one, since it is newer. I recommend the 4GB model since more and more games want more Gigabytes of Video RAM for ultra settings (CS:GO doesn't do that as far as I know) :
-2GB (http://www.amazon.com/Gigabyte-GeForce-GDDR5-Graphics-GV-N960OC-2GD/dp/B013LDXEMS/ref=sr_1_4?s=pc&ie=UTF8&qid=1451909824&sr=1-4&keywords=gtx+960)
-4GB (
http://www.amazon.com/MSI-GTX-960-GAMING-4G/dp/B00V4HY522/ref=sr_1_5?s=pc&ie=UTF8&qid=1451909824&sr=1-5&keywords=gtx+960 )
Note that while you might not see a big difference in frames in CS:GO between the two cards, in other games like GTA V and Fallout 4, the gtx 960 is quite a bit better. The GTX 960 equivalent by AMD is the R9 380, again available in 2gb and 4gb models. Choose whichever one you want. Make sure the card you choose fits in your PC and your Power Supply is enough for it. What Power Supply do you have?
Finally, do some research on these cards before buying the one you choose. You need to get an idea of which one is better and how better it is, so you can make a good choice. I'm only giving you some advice based on what I know about these things, it's up to you to choose what best fits your PC and your budget in general.
Also note that I don't have any of these cards, I am talking to you based on what I've seen from my research.
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