Can this pc run fallout 3?

Sure, but that PC is a waste. The Q6600 is an outdated, under-performing CPU, Windows 7 is a far lighter OS than Vista so it will yield better performance. 667Mhz RAM is junk. There is like one company that makes good PC 5300 RAM. Go for DDR2 800, the norm, if you can. HDD is fine. As for a GPU, grab a GTS 250, which will max Fallout 2 out.. It is pretty cheap and only needs 1 pci-e connector. That and the PCB is small so it can fit in these OEM cases. How much are you spending on this? I can probably find a better deal.