So you have two questions here. One being migrating from 32 bit to 64 bit OS. So let me answer them separately.
1) If you have 32 bit OS and wish to upgrade your OS to 64 bit OS then you should not go through the upgrade process for OS through DVD disc. You should be making the clean format the machine and install the 64 bit OS. For example 32 bit OS apps don't properly get moved to the 64 bit and there are many people who faced the issues. This is true especially in context of updates. So from that angle, you should consider clean install. If you are in doubt about this 32 bit to 64 bit OS, you should consider clean install. My suggestion: Go for clean install of 64 bit OS.
2) Second question, you may add the RAM 4 GB regardless of your OS being 32 bit or the 64 bit. In case of the system lag and other slow computer process, you may notice difference when you upgrade to 4 GB. So I'd recommend you to upgrade the RAM if possible.