hmm... maybe i can ad a atleast 2 more harddrive based players for you!
First its the Archos Jukebox wich is the chiepest(?) of all HDD players I´ve seen. Its small compared to the RioRiot But big compared to the IPOD. There have been some issues about sound quality but i dont know if they´ve solved them.
Then there is the Neo Jukebox (now bought by Toshiba) that is cheaper than the RioRiot and about the same size i think.

The Archos and the Neo are based on laptop drives wich is why they are a bit bigger than the IPOD. I dont know about the RioRiot but i think its based on laptop drives to.
The good thing about the "laptop devices" is that they are uppgradeable, atleast the archos and neo are. They have both been successfully updated to 60 GB drives.
I have lost all my links to all these drives but i think a search on altavista (or whatever) might help you!

hope this helps!

/ilDuce