CAS2 is the <A HREF="http://www.houseofhelp.com/forums/showthread.php?s=d84046d6c6c86eb841477454e727d276&threadid=5983" target="_new">latency</A> of the memory (lower the better).
What you are getting is CAS 2.5. Kingston HyperX is CAS2 and so is Corsairs XMS Series. If you are planning on getting another 512 the thats fine to use 1 for a bit, tho depending on what your doing it maybe overkill to get 1GB.
If you are not OC'ing then PC2700 is fine.
"Quotes are for people that can't think for themselves"
<A HREF="http://www.anandtech.com/mysystemrig.html?id=23900" target="_new">My PC</A>