I think a really good example to show you the best possible video card for your processor is https://www.youtube.com/watch?v=TGdo75gasaQ
it shows that the gtx 450 around the 400 series time was the best for for that processor. but if you compare it to the newer series probably something along the line of a gtx 750 ti would be the best fit for you. It requires pci-e power connectors, a really good example of what the 750 ti can do for systems likes your is in this video https://www.youtube.com/watch?v=A_Pnte_niJE
you will see some bottle necking but not as much as your would from other cards. If you have a system that allows you to overclock you could probably get a really good beneficial performance increase, if you have a good enough power supply for overclocking. the card will run on almost anything over 280 watts. I had an old p4 system with a 250w power supply in it and it ran just fine. I didn't really plan on using the card in that system I just did it as a test to see if the 7+ year power supply and 8+ year old system could run it.