The function, GetTime() returns the your computer's uptime in seconds with millisecond precision. As far as CPU and RAM usage, WoW only reports how much it is using and through script profiling, how much CPU time the addons are using.
@Ethan Yeah, I'm speaking in terms of what WoW can provide through API calls. I've got a character LCD that displays some system information, but it would be nice to see something in game. The only option I can see is an overlay, such as MSI's Afterburner On-Screen Display that shows GPU stats.
@SDPhantom I didn't realize GetTime() was system uptime.