There is no upgrade path from 32-bit to 64-bit Windows. You will have to install a "clean" installation. Also, Dell indicates the maximum memory for your system is 4GB (see link):http://support.dell.com/support/edocs/systems/dim5100/en/sm/specs1.htm
I would suggest that 4GB (assuming that is what you have now) is all that you need and can actually use. Installing 8GB, even if the motherboard recognizes it properly, will not gain you anything in terms of increased performance. That is an older system and it really isn't capable of doing more than it is now.
I would further suggest you save you money and build/buy a new, modern system rather than spending money on this Dell.