incompatible ddr ram?

hello. I ordered 512 mb of crucial ddr ram and it showed compatible with
my stystem on the site. its the ddr 3200. the chip i had in before was
also crucial but 256 and 2100. it doesnt boot with them installed together.

is this because of the different speeds? i hope i didnt somehow damage
something when it wouldnt boot. but now its working fine with just the
512 mb chip.

should i sell my old one on ebay? im a very frugal computer upgrader so im
wondering if it would be worth it to order another stick of 512. right now
im just playing world of warcraft but might try half life 2 also.


my other upgrade was adding a 9600xt and my current processor is a 1800+


thanks