Hardware Needs
'GeeXboX is an operating system but you still need something to boot on

You're lucky, GeeXboX is based on MPlayer, which can be considered as one of the less cpu-dependent movie players, thanks to the use of FFMpeg for decoding movies.
So, here's what can be considered as a typical / minimal config :
* x86 Pentium-Class CPU or above (
P2-400 should be quite enough) or Macintosh NewWorld G3 (G4 highly recommended)
* a VESA 2.0 compliant graphics card (for x86 PC only).
* an ALSA compatible sound card.
* at least
64 MB of RAM* CD-ROM or DVD-ROM drive
* Motherboard which supports booting on CD-ROM (should be ok for everyone

* Keyboard, Joystick or Remote Controller, using a Lirc-compatible IR (InfraRed) receiver (check
http://www.lirc.org to build yours), e.g. Miro PcTV's one or ATI Remote Wonder.