and would a conroe fit into the same as a pentium 4 560 as they are both 775? or would i need a good motherboard for it?
Doubt your mobo would support it. Get a 965x or the more expensive 975x board. OR wait for nVidia's 5xx series to pop it's head.
As for improvement, you should see a vast improvement in overall general applications. In gaming, it might not be much, mostly due to GPU bottlenecking. But you should still see some improvement.