First thing you probably want to get is an IOpener Hard-Drive cable. They're a little expensive and cumbersom to build your own -- sourcing the parts and assembling them just isn't cost effective unless you're doing them in high enough quantities. For single-builds, it's easiest to get parts from BadFlash.
Next thing you want, depending on your IOpener model and resources, is a BIOS upgrade that'll unlock the machine's full potential. The oldest models can be soft-upgraded with some clever hacks; the more common v4 and v5 models are locked down very well, and have no known holes to exploit via software. For these models, it's easiest to just get a new BIOS chip from BadFlash.
If you don't want to run a hard-drive, get a CompactFlash to IDE adapter, and find the biggest CompactFlash card you can reasonably afford. Keeping everything "solid state" (no motors, no fans) makes for a nice silent pc. In fact, I used to run a near-stock IOpener, loaded with an old Linux-on-SanDisk distro. That machine had the Audio Hack and served as a cheap, quiet, and energy-efficient Internet Radio receiver. I recently replaced it with an XFedora Xbox with the digital output mod, but will probably change it back soon -- the Xbox's fan noise drives me nuts.