I recommend either, based on the amount of memory the server will have. Unless you've got 4G or more of memory, or need to be sure you can upgrade to 4G or more memory, then you should use CentOS 64-bit. Otherwise there's no reason not to use CentOS 32-bit.
1 GB is not enough for x64. I recommend to upgrade before installing DA or install x32.
Besides tests you should know that x64 consumes slightly more memory.
Perhaps, but perhaps not. We still have some servers running only 1GB of RAM.l They work well for the clients who rent them from us, so there's no need to upgrade them.