http://www.yourmacstore.com/shop/shopexd.asp?id=214
to bad it's out of stock
:I
Another possibility is to add the Q3/Q7 MOSFETs (http://forums.mactalk.com.au/19/58496-g4-cube-appreciation-2.html)
I'm to bad at soldering though :-/ + i doens't clear room for a better graphics card...