what is the difference between i386 and x86_64?
i386 = 32 bit
x86_64 = 64 bit (Intel compatible, not Itanium)
at the cmd line; it'll show whether you're running 32 kernel or 64.
The install will likely choose correctly, but no harm in checking first in case it asks.
For rpm (assuming v5.x you'll be using yum hopefully) the pkg mgr eg yum will know what you've got and install the correct versions.
RHEL is a dual-lib system, so even if you've got 64 bit, some progs will still be 342 bit and RHEL will load the correct libs in the correct place as needed.