There were dual-core 32bit processors, but the Pentium D is 64bit.

The motherboard may limit the amount of memory (RAM) that it will take. We'd need to know the make/model of the machine to tell.

Gamers often upgrade their graphics cards to the latest and fastest so you can pick up the not-the lastest at places like CeX. Any competent 13-year old should be able to swap one.