Personally, I would only install the x86_64 distro if I was reasonably sure that I would not require i386 RPMS (or minimal i386 RPMS).
I just use i386 on all workstations and I use x86_64 on servers ... and even on servers, only ones that will really be under heavy load or will definitely not need i386 packages.
What are advantages of 64 bit OS anyway? I was thinking with i386 the max RAM you could have was like 4 gigabyte or something? 64 bit allows quite a bit more, right?
I am upgrading a very heavilly used email server to a AMD64 dual core with CentOS. I am staying with i386 since the web GUI we use lists 64bit support as beta and I do not want any problems. The real draw back I see is the max RAM on i386 but perhaps I am wrong. 90 percent of our CPU load is Spamassassin. Disk I/O is likely a big bottle neck as well. Currently we run 2 gigabyte of RAM but will likely move to 4G of DDR2 RAM. Moving from PATA to SATA drives as well.
Matt