I think that you would benefit most from an entire rebuild if its in the budget. Only because a more modern graphics card would most likely be limited in its performance by the older CPU and upgrading the CPU is not going to give substantially more performance out of the older graphics card.
However on the cheap side, you could upgrade both the CPU and the video card, thank AMD for small favors, this CPU *should* work on that mobo:http://www.newegg.com/Product/Product.aspx?Item=N82E16819103727
And a video card upgrade could depend on your budget.
Note: the 965 is not officially supported on the AsusM3A78 board, however, at least one person has managed to get it working:http://www.tomshardware.com/forum/303880-30-will-asus-phenom
Based on their support list, they clearly support C3 stepping Phenoms, theres no reason it shouldn't.