Well this is all my opinion.
I wouldn't use the Nvidia 9400GT becuase its only clocked to 1.4Ghz, Over clocking it wipes your warrrenty if any, but you maybe able to clock to 2Ghz.
I think a new graphics card maybe needed. As this graphics card is a bit old, your mega multi core processor wouldnt be running at full because nothing else can keep up.
If you are running several visual programs or editing software, ram and your processor is more ideal rather than graphics card, it helps dont get me wrong. As you said your computer only has 2gb ram. If you are running visual based software/editing programs you might want to up it to 4GB, and even with gaming you would want to do that to. I run 2gb for World of Warcraft, it runs fine at times but when it comes to stepping out of wow, to google chrome it 'lags'. This is due to insufficient ram. I believe you would experience the same kind of lag running visual based software at some points.
Depending on what games you play will depend on what specs needed for a graphics card, due to so many games needing different gaphics specs. Such as, COD, i cannot play it with my graphics card, and same with some other games, but i manage wow nicely.

hope this helps.