Mini ITX?

it looks like you only need a motherboard and some memory, but thats too simple.
Like any computer, you need a CPU (which may be on-board, or there's a socket for one), memory (which you'll have to add), video (usually on-board, and difficult/impossible to upgrade), and storage. Which might be 2,5" laptop HDD, solid state disk, or even CF card. It all depends on what components you pick & how you use them.

Other than that it's just an ordinary PC motherboard in small form-factor, which may result in some limitations. And building a complete system may be quite a puzzle: where to get the parts, will things fit together physically, how to arrange power supply (and is it strong enough, internal or external), cooling, etc, etc. OTOH: figure this out & you can put very nice & small system together. See for example mini-itx.com or silentpcreview.com for some ideas.

I have no experience with Mini-ITX myself, but there's a good chance my next PC will be based on it - currently have my eye on some AM2+/AM3 Mini-ITX boards from J&W (Minix series) and Asus. Mostly because I don't do 3D games that much anymore, Mini-ITX boards have become more available & affordable, and it's unbelievable how much compute power you can pack in a shoe box these days. :) Most important missing part right now: the budget... ;(

C64 case is small, so consider to get a ultra embedded board as Via nano-itx or similar (note that performances are low when compared to today's standard PCs). However internal airflow and emf shielding may be issues. Moreover, i don't know how the c64 keyboard may be adapted to standard pc usage.
please let me know, me too i'm building an itx case, so i'm interested

Aurora: all of the above! With those specs, I prefer the Shuttle X350. The 1-liter PC. :)

I just came across this thingy: Commodore PC64. Dunno whether to :D or :-? or oO or :RNFF:And now it's gone from the Crommodore website, but there's still this thing. And that's one ugly mofo :o