What is the best upgrade/s i can make to my PC for modern gaming ?

It looks like you can probably throw an AMD Phenom 965BE CPU on to that motherboard. You would want to do a little more research to confirm. But if you can pick one up for a decent price along with an upgraded GPU you should see a pretty good performance increase.

You might want to start with telling us your budget along with your PSU like the previous poster has asked. Then we can give you a more accurate suggestion.
 

clutchc

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This GTX 750 Ti would be a good match for the Athlon II quad core. But you need to verify that your 300W PSU has the +12V rail capacity to run it. That's why we need the nameplate specs. Can you open your PC and see your PSU's nameplate?
http://www.newegg.com/Product/Product.aspx?Item=N82E16814487024
 

clutchc

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Forget it. That's not a very good PSU. And not enough power for the card I mentioned above. You'd have to upgrade the PSU too.