Firstly, buy a core 2 duo. It'll whoop the pentium D any day.
Now onto the graphics card, I recommend this, an XFX 7750, it's 100 bucks and is a fantastic low profile card that handle your gaming needs. It shouldn't bottleneck if you buy that core 2 duo I mentioned above.
The Pentium D and Core 2 Duo would get extremely similar performance results, no need to buy a new CPU.
As Diox said, the 7750 is about the strongest low-profile GPU you can get, and at about $100 it'll give you pretty good performance too.
Honestly though, most games are going to require a core 2 duo. It might not be mandatory, but comeon for 20 bucks? Pick it up while it's cheap so you don't have to get it later.
With both having 2 cores, the performance difference for gaming will be the clock rate. The Pentium will perform just about evenly with the Core 2 Duo.
CPUboss and GPUboss don't accurately portray real-world use, especially not specific real-world use. The Core 2 Duo will perform better in regular programs, but will not be noticeably or materially better for gaming.
With both having 2 cores, the performance difference for gaming will be the clock rate. The Pentium will perform just about evenly with the Core 2 Duo.
CPUboss and GPUboss don't accurately portray real-world use, especially not specific real-world use. The Core 2 Duo will perform better in regular programs, but will not be noticeably or materially better for gaming.
I guess we'll just both have clashing opinions. I've always seen and experienced better performance in a core 2 vs pentium D.