The P4 is a fake duel core (hyperthreading), while the PD is a real duel core, so for most programs there will be literally no difference at all. But for games which do not take advantage of hyperthreading, but do take advantage of multiple cores, it will give you an approximate doubling of processing power. But with the other bottlenecks involved in your computer (ram, bus speeds, gpu, etc) there would be other issues preventing you from getting much out of your games. My guess would be roughly a 5-10% increase in performance, which is going to be barely noticeable. So for $20 it may be worth it, but to be honest I would not pay more than that for something that will do next to nothing.
A similar, though larger scale, example of this is my attempt to go from a C2D to a C2Q thinking I would get an approximate doubling in performance out of Adobe Premiere. In reality there was next to no difference when it came to editing the footage. The bottlenecks in the ram amount, ram speed, GPU, and system bus were just far too much for the platform to handle. In the end I decided to upgrade the platform, and have been very happy, but I completely understand that when the money is not there then it is not there.
Your largest change in the upgrade would be the use of less power moving from 90 to 65mn. Other than that it would be a bust.