A Pentium D is an older CPU. To determine if it is even possible to upgrade to a new PC, we need to know which motherboard you have. If you don't know, you can download CPUz here
. The Mainboard tab should list manufacturer and model. However, if you've already upgraded your RAM to the maximum capacity your PC can handle or the that CPU you have now is the best you've got, you may want to consider buying, or better yet, building, a new PC.