Spin speed is important.
Most hard drives that fit inside dekstop PCs (3.5") spin at 7,200 rpm.
There are some that spin at 10,000 rpm but they tend to be more expensive.
Cache is also important, the more the better.
It starts at the low end with 2Mb, then goes up to 4Mb, 8Mb, 16Mb and 32Mb (maybe higher but 32Mb tends to be the normal top limit)
Noise is also immportant, so you want a quiet hard drive if you are going to watch movies.
Just as an example, here is a 750Gb drive that spins at 7200 rpm with 32mb cache for �61.
http://www.ebuyer.com/product/139739
Not saying it is the best, but it gives you a starting point.