To give you some personal input, since you only have 2 gigs of RAM, upgrading from 32 bit to 64 bit wont yield much of a difference, the biggest noticeable difference between 32 and 64 bit is the removal of the memory limit in the 64 bit, since the first 64 bit processors finally began utilizing more than 3 gigs of RAM. The rest of the pros to 64 bit OS is in the processor itself, and the Pentium D is a 32 bit native processor, so I dont know exactly how well it will run it in a 64 bit enviroment (my research tells me it is only able to run a 64 bit enviroment through emulation).