A warning for anyone switching to a 64-bit architecture (such as AMD64) from a 32-bit: Subversion repositories have to be migrated (if you used the default Berkeley database). This means you have do a dump with on the 32-bit system, and then a restore on the 64-bit. You can't just use the same files.
I discovered this the hard way. I hope I haven't corrupted my databases! I'll be switching back to i386 very soon. I usually set up my machine so that I have a '/home/' partition, and can share this between different installed distributions. This can be problematic if you have different versions of programs with incompatible config files for example, but with this Subversion problem it's much worse. It means that although in theory there is very good backwards compatibility between AMD64 and 32-bit chips (AMD64s can run 32-bit binaries), in reality things are much more complicated, and you have to really treat AMD64 as a completely different architecture if you want to be safe.
By the way, you'll have similar issues using 64-bit Windows from what I know (e.g. 32-bit plugins won't work in 64-bit browsers, these kind of file format problems could well exist if the developer didn't take particular care to avoid them etc.). The world is not very 64-bit friendly yet.