
There's only one physical size for VRAM SIMMs (unlike DRAM SIMMs which 
come in many, many sizes and pinouts), although they do come with 
differing amounts of VRAM on them.  The ones you need are 256KB SIMMs, are 
organized as 128K x 16, and have two 128K x 8 VRAM chips on them.  This is 
the only size which the Quadra and Centris machines can use.


Two.


Both SIMM slots must be filled; putting a SIMM in only one slot does 
nothing for you.


80 ns for the Q800 and C650, 100 ns for the C610.


There are certain VRAM chip manufacturers whose parts are not compatible 
with the Quadra and Centris video hardware.  Make sure that the source you 
get them from guarantees compatiblity.  In general, if it works in a Q950,
it will work in a Q800.


Trust me.  ;-)