any reason to NOT use 64-bit version of java?

[abc_18]

We're starting to use big heap sizes (as in
"java -Xms2048m ..."), and may need/want to
go larger.

For 32-bit java, the cap is 3.8 gig.

Q#1: what's the upper limit for heap size with
64-bit java?

Q#2: are there any drawbacks with running
/opt/java/bin/java in 64-bit mode (ie, with
the "-d64" flag)?

Thanks in advance.