Hmm... there are any number, but none are cheap!
For example, have a look at 'Advantech' (Taiwan) and 'WinSystems' (US, www.winsystems.com
I use a lot of WinSystems gear; the gear is great and they're a very pleasant company to deal with; they look after their customers. (I'd better not say too much about Advantech, I might be asked to edit my post.)
You probably want an x86 based one to make life easier.
Now the bad news: those things cost between USD300 and USD1k, plus you have to add your own power supply ($80 for a really good one from 'MiniBox' - the 'Pico' PSUs are awesome), case, Wireless card, etc etc etc.
Now if you can get one of these cheaply it would probably be good enough for you:
Also check out 'Mini ITX' and 'Nano ITX' boards and cases; if I remember correctly, 'MiniBox' have some pretty good items: http://www.mini-box.com
Like I said, their power supplies are among the best. MiniBox also usually offer at least one type of case for whatever MoBo you find on their list, and if you're very lazy you can throw some money at them and they'll assemble and test it before posting it to you.
My guess is for a case, memory, at least 80GB HD, wireless, MoBo, power supply, you're looking at about USD600. It's certainly no toy though - it's a very capable machine - you'll have something of a 'Super iPod' - really, these machines have far more computing capacity than an iPod, so feel free to go for the slowest lowest power and cheapest.