On Xen systems you can also set the CPU weighting, which is used when the total CPU consumed by all instances exceeds that provided by the host. An instance with a weight of 512 will get twice the CPU of one with 256, assuming they are both doing equal amounts of processing.
Warning - reducing the memory of a running system below what it is current using will almost certainly cause it to crash.
On Xen, Zones and Vservers systems, when Cloudmin bandwidth monitoring is enabled, you can set the limit on the amount of data traffic that this system is allowed to consume over the current accounting period (typically the calendar month). Exceeding this limit will trigger an email and possibly shut the system down.