To upgrade or not to upgrade

I have a Dell Dimension 4400 with 1.6 ghz Pentium 4 processor, 768 mb of ram, a GeForce 2 graphics card, an AGP connector, and a 100 gb hard drive.

I was thinking of switching my old processor with one of them fancy dual cores or replacing my GeForce 2 with one of them fancy GeForce 7s. Would either of those upgrades yield noticable performance gains, or would some part of my old motherboard limit the utility of those new components?