Power Unit Supply (PSU)?
Need to know brand and model.
At first glance, I would upgrade the GPU, given the PSU is decent, since it is the single piece in this case that will yield the best performance gain.
The i5 650 is an old CPU but still decent if paired with a decent GPU.
300USD for a GPU, I'd probably pick a HD6950 or a GTX560Ti and save the rest of the money.
If you need to upgrade the PSU aswell, use the rest of the money to buy a new one, XFX550W,650W, Corsair TX650W, between other good picks.