All Pentium Ds are dual cores. They are just 2 Pentium 4 processors on the same die.
The CPU is your main problem. A 3Ghz Pentium D is roughly the same as a 1.4Ghz Intel Atom netbook processor. Not going to cut it for any game newer than about 2006. Your graphics card is lower end as well.
Nothing you can do to upgrade that system. You need a new motherboard, CPU and RAM minimum.