What happened to my 1GB RAM? Is there a way to check? Thanks.
It's still there, and free to be used. The '762112K cached' indicates that 762,112kB of your memory is being used as a filesystem cache -- this is what the Linux kernel does with free memory, rather than just leaving it free. The kernel will free it up automatically when it's needed.
So in reality, you're only using about 250MB of RAM -- if the machine is being used for hosting or something, that's probably not too bad.