RAM upgrade for Dell Laptop?

[smccord313]

I currently play PS on my laptop. It is a Dell Inspiron 8200. My specs are:

2.0 ghz p4-m
512mb DDR pc2100 RAM
ati mobility radeon 9000
win XP home
40gb hard drive
DSL connection w/laptop connected through 802.11b wireless network

It slows down in bigger battles and it is difficult to kill other people when it slows down in firefights. It runs good when there isn't too many other people around, but it sometimes skips a frame or two there as well. I have most settings turned off and run on low.
I think the problem is that i don't have enough RAM. I think the 512 RAM in it is all one stick so the other slot is open to another stick of 256 or 512.
However, the Dell website has up an absolutely outrageous price of $215 (yes this is not a joke) for a stick of 512 for an Inspiron 8200. (i could buy a freakin 9700 pro for less than that) Please tell me there is some other place to get RAM for a laptop, becasue if I'm not mistaken, desktop RAM won't work in a laptop. (or will it?) I can't afford that much for more RAM and would really like to play PS without slowdown.
Another question I have is how do I know if my processor and RAM are at the same speed? I've heard that if they are on a 1:1 basis, it runs faster. How can I check this to make sure they are at the same speed?