I've upgraded a few laptop CPU's P4 or later but would think the same rules apply.
Get CPU's that can be returned. In theory researching the laptop and also the chipset will give you an idea of the fastest CPU, clueless1 is anything but this and given good info already
BUT
the reality is sometimes the laptop will run with a much faster CPU as whoever never updated the spec sheets or it may go the other way and not run at all due to lack of BIOS updates, motherboard revisions or maybe even was done intentionally as the cooling isn't upto the task for hotter CPU's
I've retuned a few CPU's just be honest and say it doesn't work with your system and I've never had anyone refuse retuning the item for a refund.
You didn't say but if you haven't max out the RAM, especially if your trying to run WinXP on a P3, It'll love you for it.
You could also try replacing the HDD, laptops usually have slow RPM drives to save on battery, probably not a concern anymore.
You could also try Win2k rather then XP to squeeze more speed out of it. P3 was right at the crossover so makes a fast 2k system or a slow XP system.