It really depends on what type of programs you are running. If you're running programs that only require 512 mb (I assume you meant 512, not 521), then you wont see a difference after upgrading. If your programs require more than 512, then you will see an improvement (how much improvement depends). What type of laptop is it? What are you using it for?
You'll see a difference jumping up to a 1 GB of memory. If you're running XP, 512 is the minimum that I would run. Programs will load much faster and when you run multiple programs, it will really help out. If you run lite programs that already load fast, I doubt they will load much faster. It's an upgrade thats well worth it IMHO.
oops, i didn't see that you was refering to a laptop. but, you still can.. but since it's a laptop, you shouldn't. for now, memory upgrade is possibly the only way to go.
The actual swapping out of the cpu isn't that much different if you have done it on a comp, you could probable google a guide for you comp, but a 1.8GHz p4-m is one of the faster mobile cpu's out there, its not very slow. Now it is a little tighter in a laptop, everything is crammed together so it will be a little more difficult, but if you have any doubt don't even begin to attempt it. I would upgrade the ram first, and then if it is still slow, then go with the cpu.
i just got another 512mb for my laptop and now i have 1024mbs total. its weird because i dont see an improvement at all, some stuff seems like are running a little bit slower?