Should be able to put another 512 in but it needs to be the same- although I have used mismatched memory before and it worked. Download cpu z and check your memory on it for timings etc.
CAS Latencies are usually written as CAS2 or CAS3, so just how important is this?
In the real world, unless your system is up on the cutting edge of technology and you're pushing performance to the limit as do some over-clockers, or gamers, it may have some relevance. On the other hand, in everyday systems the relevance is nominal at best. CAS3 means that at 100 Mhz., the amount of time required for the first memory access in a burst is increased by 10 nanoseconds or less. Divide this figure by 4 to average the increased time across four bursts, and you have an improvement of less than 2.5 nanoseconds over CAS2. We need to underscore the term relevance as it pertains to CAS Latency and changing memory modules on the average system. If you had a Pentium III 600 to 866MHz. computer, as an example, and you used this for surfing the Internet, using Microsoft Office or Corel Office, Adobe products etc., and changed your memory modules from those having CAS3 to CAS2 latencies, you wouldn't be able to notice any difference. But again, if you are pushing your system to the limits, this could become critical.