In regards to your budget I believe your upgrade priority should be PSU>GPU>RAM
Apologies the only place I know about PC parts from is Newegg as I'm not American so I'll be using those links.
Ram> Buying it in pairs is always best, hence why I've linked a pair of sticks. Something like this would be a fine upgrade for your RAMhttp://www.newegg.com/Product/Product.aspx?Item=N82E16820148677
PSU> This part largely depends on the upgrade path for your chosen GPU but due to a potential bottleneck with your processor that is limited so i think something like this will be good http://www.newegg.com/Product/Product.aspx?Item=9SIA3KX16D4793($100
GPU>Due to the limitation of your Processor I think something like this would be a good upgrade from your current card http://www.newegg.com/Product/Product.aspx?Item=N82E16814127663($130-150)
That said you could also benefit from a change in processor but I've just tried to do the best in your budget
Alternatively you could just pick up a new processor like this one instead of the GPU http://www.newegg.com/Product/Product.aspx?Item=N82E16819116781
and then in the future just pick up a GPU as you can accord, the current one you have is not awful but not exactly good either.