I watch CPU, RAM/Swap, and Net I/O. CPU and memory because sometimes apps start to behave badly, and I take great delight in terminating them in the task manager when they do so. Net I/O because I like to know if it's my router/wireless adapter, my ISP, or the DL site that's making things go slowly. (I can do independent checks of the first two...).

I use xstatsgraph/xstatsclass for everything these days.