Depends, (AMD VS INTEL) not in performance per dollar for example.
Anyhow, I'd upgrade to a motherboard with AM3 support.
There are several cheap choices, I'm using Athlon II x4 640 3.0 GHz. However! You should buy Phenom II instead of Athlon II because of the L3 cache which performs better.
For the RAMs, I'd get Corsair XMS3 DDR3 1600 MHz which gives incredible performance for a low price, (most gamers use this memory).
Could you give the full speccs of your computer and we could help you out further? And what are you going to use your computer for?
And if you're going to use the processor for games, I'd like to know your graphics card. Since most 4 core processors won't slow your graphic card down, unless you've got like GTX580/Radeon HD 6990 which I don't think you've