What kind of Internal hard drive should I get?

Your box says SATA II so any SATA HDD you can afford will work. The Blue and Green drives usually have a bit slower performance. You will just have to look at your suppliers to see what you might want, HDD's have become very expensive the past few months.

The SATA drives will run at the slowest standard of the HDD or MB. A SATA1 drive will run @SATA1 speeds even if connected to a SATA3 motherboard. A SATA3 drive will only run at SATA1 speeds if it is attached to a SATA1 motherboard. The SATA standards are backwardly compatible, they just run at the slower speed.