As with any memory, frequency, timings, and voltage.
For capacity, it's just what you need. If your usage will exceed 4GB, then you should get 8GB.
Higher speed is more OC room in a sense, but most likely it will just give you more memory bandwidth.
Depending on your motherboard, DDR2-800 CL4 is a good performance spec that most can handle. Then there is DDR2-1066 CL5, DDR2-1100 CL5, and DDR2-1200 CL5 which is the top. The only differences between them all will probably be the voltage. The lower the voltage the better.
For DDR2-1100+, most companies will be upwards of 2.10V. I would try to stay under since it can become very stressful on the Northbridge. DDR2 is limited now, so anything2.10V and below would be good enough. G.Skill use to have a Trident series 1200 @ 1.80V, that was crazy in the day, but probably very difficult to find these days.