Yeah that sounds about right. If I were you and you can afford it, I would go for at least a 512mb stick, you should notice quite an improvement for that, 256mb is really quite low for XP.
As for installing it, it's not that hard to install ram into a laptop, check your manual to find out where you access your ram slots (normally by unscrewing a panel on the underside) - One hint here - make sure your laptop is unplugged and that you have removed the battery before you do this. Unlike installing desktop memory sticks, laptop SODIMMS are inserted into the slot at a angle then pushed down parallel with the motherboard until the latches engage. That's it. Close it up and put battery back in. Just remember to ground yourself before touching the memory stick or the inside of your laptop by touching a radiator or similar to avoid the slim chance of static electricity damaging and internal components.