If your running KDE, there's the hardware browser under the "system Tools" menu. At least it's there on Fedora Core 2. Also, the KDE "Info Center" applet under the same menu has some stuff.
From the command line:
Memery Stats: free
CPU: cat /proc/cpuinfo
Info on various PCI, AGP & USB devices: /sbin/lspci
Temperatures, fanspeeds, etc. Look up lm_sensors. Gkrellm has an interface to this.