Dell 2400 performance question

JJO Adding more RAM will generally help you with mulbreastasking unless you're running something very specialized that needs large amounts (doubtful).

A faster CPU will help you with speeding up specific CPU intensive operations. For the most part these are: encoding audio, transcoding video, compiling, extremely intesive math calculations (eg. CAD models), and games. You'll see a slight bump in some everyday tasks, but it will generally be insignificant unless the CPU jump is substatial.

The P4 is a much better designed CPU than the Celeron, but in terms of bang for your buck, it probably isn't worth the jump for less than 500MHz. One thing to consider is picking up a CPU-mobo combo and performing a swap. I was at Fry's yesterday with my friend and he picked up a P4 3.0J (800MHz FSB, 1MB cache) wan ECS motherboard (supports PCI-X & AGP as well as DDR & DDR2) that uses the Intel 915 chipset for $250 (no rebates). I beleive you'd notice a jump with something like that, but just going to a P4 2.66 w512KB cache & 533FSB would be suddle.