I have a P5K-E and use Transcend DDR2-800 (model no. JM2GDDR2-8K). Its very stable and very well priced. If you check the model number on newegg.com you'll be able to see lots of user comments. On the downside there are no heat spreaders but I have not had any probs in terms of temps. I am running a Q6600 overclocked to 3Ghz. The ram is running at 1:1 ratio so is actually only running 667 at the moment anyway, but I am currently looking at either lowering the latency (5-5-5-12 to 4-4-4-12), increasing the ram speed so it runs 4:5 (Ram faster) or both. Both options work - I just haven't tried them together as yet.
If you are really going to push the system to its limits then I wouldn't say that this is the best but average overclocking or running at stock I would say its excellent value for the price. I was actually going to buy the Corsair myself originally but the price went up here so after seeing the reviews of the Transcend I bought and have been very happy with it.