Yes you can install another RAM to the second slot but first check your computer's manual to see how much amount of RAM it can support. Probably it will be minimum 2 GB but you should check it to be sure.

It's not advisable to use different RAM speed, it is possible to face stability problem,when you buy the RAM you should try to get the same chipset and brand to have RAM compatibility.

Our advice is to use the same amount of RAM on the two slots and the same brand and frequency to have 100% compatibility. Your configuration will probably work but there is a small chance to not be very stable.