I would recommend the following stick of memory:
http://www.newegg.com/Product/Product.asp?Item=N82E16820141062
These are my reasons for it:
First of all, your board runs on the PC2 6400 standard, by going with this memory, you would remain in spec.
Second, this memory is not much more expensive than the memory of the next speed grade down, so there is no reason not to get it versus saving some cash on slower memory.
Thirdly, it is PQI brand. PQI is known as a quality brand, so you can be assured that you will have no problems.
Fourth, I suggest this one 1GB stick instead of getting two 512MB sticks as 512MB sticks are pretty much out of date. If you purchased a single 1GB stick, you would still have three more memory slots to upgrade later on to. Also, the performance lost from going with single channel at least temporarily is not significant.