I would choose the 64-bit because it's the future of computing and a lot of new software that is coming out later might only be written for 64-bit.
May I ask what made you purchase a GTX 260 to go with your Pentium D????? XD
But seriously if you want to upgrade to win7 I highly recommend you get a faster CPU than that. With a processor that slow I would be surprised to see you with more than 1GB of ram which is the absolute minimum amount you want to be running win7/vista on.