I agree with Nagus - get rid of the current RAM and replace with 2 hi speed sticks, go for the fastest your motherboard will work with (even if it won't run it at the max speed - then if you do decide to upgrade the motherboard & CPU you won't have to buy new memory).
I would recommend you ignore the Crucial websites' advice about changing to a 64 bit version of Windows, as it's debatable whether there would be any noticeable gain in speed with your current system - and that's before the problem with getting all the necessary 64 bit drivers for all the system components.
I'm currently running a dual core 4800 processor with 2 Gbytes of fast DDR RAM and Photoshop flys even when I'm editing video and writing DVD's at the same time.