Dell Inspiron 8200- memory inquiry & compatabilities

Hello to all,
I need help with the memory decision, and would appreciate any help.

I at the moment have a 256mb stick and a 512mb stick.
Dell's specs say the i8200 can only handle 2x512mb sticks, but many others on the web say that Dell's specs are old and many have 2gb running in them with no problems. I know I need to upgrade because I do editing in PS and see the commit charge go to 1gb+. ANyways, So I'm looking for a 1gb stick to use with the installed 512mb to get 1.5gb.
My situation is this. My neighbor wants to sell me his 1gb stick. It's pc3200.
I know the laptop will work with 1gb as many others say it will work, but is the memory backward compatible with a pc2100 system? What problems besides not recognizing can I have? Can I just install it and see if it'll work, or do I run the risk of blowing it up into a fireball?
also, will it have problems if I put 2 different spec'ed sticks with different sizes inside?

Thanks