Is it true that Logic wont recognise more than 4 gigs of RAM at 32 bit mode?
Yes... no application can access more than 4G in 32 bit mode (and Logic doesn't like using more than 3 generally).
The solution these days is to run in 64bit mode, with as much RAM as you can sensibly afford.
All the info is here:
Note that MacPro's tend to perform better with the RAM in matching sticks (ie all the same), and, depending on the particular Mac, they work best when the RAM is installed in groups of 3.