memory card compatibility

[looksalot]

I have 2 pc's that could use more memmory. I went to the store and bought 2 - 512mb cards, installed one in my (relatively newer) machine just fine, works fine.
I then went to install the other card in the other machine, and had to go back to the store to get the correct card - needs 2 "notches".
My question is - Where can I find out if the card with 2 "notches" will work in the machine that only requires one "notch"? (Machine that needs 2 "notches" has 3 slots now containing 64mb, 256mb, 256mb - will pull the 64mb and insert 512mb giving 1gb. Machine that needs one "notch" has 4 slots containing 256mb, 512mb - want to add the 64mb card.)

[ramasees]

i think that =
2 notches is sdram which i believe is phasing out
1 notch is ddr which is used now
ive found that its not a good idea to mix memory differnet brands and sizes as this can result in unpredictable random errors