You can now buy a MacPro from the online Apple Store with a whopping 64 Gig of RAM pre-installed. You are probably going to need as much RAM as possible when the new FCP is released, so start saving!
I think before this you could install your own RAM up to 64 Gig, but you would have had to have thrown away the 6x1GB memory the machine was supplied with. Now Apple are selling a fully RAM loaded MacPro as a BTO option, an expensive one though.
FCP has always been pretty poor at memory management; if you've worked with a lot of stills or plugins within FCP then the 'out of memory' error message is no stranger to you. Looking at how Motion eats RAM, you can be pretty sure that the new Cocoa 64bit version of Final Cut Pro is going to be as RAM hungry as its younger brother.
For all you tekkies out there who like a bit more information, a bit of Googling shows the likely candidate for the RAM is the Samsung RDIMM 'Green' DDR3 DRAM which has 50 higher memory density and a low power consumption. I don't think you are going to find those at Maplin though!