how about a hd 7750 it's low in price, but i do suggest an increase in ram
when I'm doing my work on my pc + lots of stuff open, firefox or google chrome, and lots of editing and cad with over 200 layers and other stuff then rendering left and right, you will see huge usage lol but anyways in your case, i doubt it's the CPU but if upgrading the ram doesn't fix this i suggest doing a pc clean up like defragment or using ccleaner or maybe you got a virus in your pc, is your anti virus updated? if it is then disregard that part of my post, if not then you need to update it and if it is do a full scan, if nothings shows up then it's a mystery my sisters pc opening lots of tabs around 35-40 doesn't slow down as much with 8gb ram, using i7 2600k @ stock clock whenever she's doing lots of research so there might be some problems with your system or maybe your anti virus is slowing down your pc which happened to my older pc on certain anti virus titles and if not try using google chrome or fire fox