Wow, you don't need a better psu.
My friend always laughed at tech forums on the psu advice, but now I'm laughing to. It's a cheap pc, he doesn't need anything "reliable" i mean most case psu last a good while, look at my rig, i have my case psu and it's fine.
I would say yes to the 8400 since it would help a bit and its really cheap.