Laptops tend not to be very upgradeable, I scarcely see them otherwise. You can sometimes upgrade the CPU with one from the same series, but no other, you can add RAM and change the hard disk, the rest are usually pretty model-specific and unchangeable.
This laptop has a powerful graphics card, but is let down by a mediocre screen and a slow processor. If you are able to swap it out for a 2Ghz new model chip, then by all means do so, but you ought to find out if it's compatible first, that's not often the case with notebooks.
Sounds like you've done your research. Go with the highest CPU you can stick in there really... Make sure it's compatible first, then stick it in. The 9300 is a reasonable amount faster than the 8300, and the same goes for the 9500 vs 9300.
