Putting a machine together piece by piece with your own hands has largely been the realm of PCs, with Mac users more or less content (or perhaps stuck) with prebuilt solutions. Apple's incorporation of PC architecture hardware has opened up a new realm of possibilities for Mac hardware (some of which Apple does not like). Thus we are now seeing more vendors catering hardware for Macs, and recently Corsair announced 4GB memory kits aimed at the Mac Pro.
The units are FB-DIMMs operating at 800MHz and carry with them Apple certification to be compatible. The hardware comes at a cost, with a 4GB kit running a price tag of $250, but that is only marginally higher than other FB-DIMM kits of that size.