It'll be processor, I like the old athlons but they do struggle with the latest. Just to make the point I've just 'upgraded' from a Quad core 9550 Phenom to an Intel Q6600 quad.
( I LIKE classic computers, alright ! )
There really is a huge difference in gaming speeds, loading times etc, and knowing how quick a good i3 or i5 is you really can't compare yours to an i3.
Either spend up on a new one, or, like myself, pick up something decent secondhand.