The >4GB problems with 5.3-RELEASE are a well known problem and affect
for i386 and amd64. It's pretty much fixed in the 5.3-STABLE stream,
but there are a few edge cases that I still need to fix which prevent me
from feeling good about turning it into a 5.3-R errata fix. It'll
definitely be fixed and fully working for 5.4-RELEASE next month.