I have a server with 128 MB of RAM. Would it be stupid of me to install Debian 6 x86_64 instead of i386? Would there be any difference in how well the server performs?
There is a very minor memory overhead with x86_64 binaries. This is usually negligible on all modern systems, so the standard recommendation is x86_64. In this case, though, I'd really stick with i386. 128MB is nothing and you'll need to squeeze every drop that you can get.