Unfortunately, your motherboard and processor are several generations out of date, there aren't any new processors you can get, they have been discontinued a couple years ago. Current processors aren't compatible with your motherboard. What you can do depends on how much you want to spend, your post sounds like you don't want to spend too much.
Getting a new processor and motherboard right now is fine, you won't be stuck with old tech. Intel's latest processors just came out about 4 months ago, and processors don't become obsolete for at least a few years assuming you spend the money to buy a decent model (like you did with your i5 760).
That said, it looks like your current processor will run TW3 just fine. This
youtube video and
reddit thread show that it is possible to achieve at least 40FPS on fairly good settings with your processor. Lowering the settings could allow you to get 60FPS, but by how much I'm not sure. What's holding your PC back is the old graphics card in there, the GTX 460 is not very capable and
might only barely handle TW3 at the lowest settings
and resolution. That said, it's up to you whether you want to wait for next year's graphics cards.
.