New processor?

Ok, my question is this. My Compaq Presario CQ50 110-EM, has 3 GB of RAM and i want to upgrade it to more, but i have a 32-bit vista, and need 64-bit vista for more than 3.5 GB of RAM, but in order to get the 64-bit vista i need a processor for it that can run it, mine at the moment cannot so i need a replacement, where can i find a processor that can run 64-bit vista and be compatable? and where can i find a version of 64-bit vista?