if you are not going to overclock much i would suggest you get any cheap cooler master or arctic silver paste it will definitely be enough . however you also need to make sure you apply the thermal paste correctly so check some videos on youtube if you are not sure on that. some people prefer a thin layer of paste spread over the entire cpu surface and some may argue that a dab of paste on the center of cpu is best but do some research and chose the best method. no matter how good your thermal paste is it cant be useful unless applied correctly. i personally use the cooler master ice fusion and i have spread a thin layer of paste on my core 2 duo after overclocking it from 2.93ghz to 3.5ghz my core temps are still at 70c max even while playing games (metro 2033).
All of the comparisons I have seen between thermal pastes showed only a few degrees between one and the other for the most part so that's almost not an issue. The arctic silver brands always hold up well when it comes to drying out from what I have seen and I remember many of the comparison showing the AS5 near the top so I would go with that. I'd say how you apply the paste will have more influence on your temperatures then the paste itself though.
Tuniq Tx-4 is a great thermal paste if you can find it. I think the best way to apply thermal paste is just putting a small amount in the center of the processor and let the heatsink spread it as you install it. There has been some test as to the best way to apply thermal paste and the pea or rice grain technique shows the lowest temps.