Although answer has been accepted, there's a lot of stuff missing from the discussion here.
First: Alexandru does not mention what database engines are running on this machine, nor whether it's a dedicated DBMS server or if it's also running other stuff, nor what size the data is, nor any stats on physical I/O / VFS usage.
For a dedicated MyISAM based server, then you're not going to get much benefit from making more memory addressable. VFS does as good a job of caching the I/O as MySQL can. OTOH if it were innodb - then by golly, yes: the innodb buffer pool on a dedicated box should be around 80% of the phsyical memory on a dedicated machine.
Regardless of which DBMS is used, switching to 64bit has significant a performance benefit when doing comparison operations on the DBMS - how much depends on whether you're doing a lot of hash or b-tree based lookups.
If the same box is being used for webserving, then the big difference will be if you're using a lot of SSL on the box - SSL is all about calculating large numbers - so even without the AES acceleration in recent intel chips there's an improvment there too.