I'll take a slightly different approach here and caution you to consider what applications you are running before you go to the trouble/expense of adding more RAM.
While adding more RAM is a usually a good way to improve your system's performance, I've noticed that unless you're frequently using applications which you know will benefit from having a large amount of RAM you may not see much benefit.
A while back I increased the RAM on my Core 2 Duo based system from 4 GB (what you have) to 6 GB. Frankly, I think almost all of the time Windows 7 just doesn't use the extra RAM. In my case with the apps I'm running (mostly watching video or web surfing) the 4GB I already had was sufficient.
So stop and ponder a bit more before you leap. Depending on what you do on your system you might actually benefit more by spending the money on another component.
FWIW, in the past I've purchased my RAM from newegg. My approach was to look over the offerings, find the ones that matched my system, and then wait to see how prices went. I've had good luck with Crucial and not entirely bad luck with Corsair. And as was already mentioned, the most important thing, aside from getting the specs right before you order, is to go with a supplier which warranties the memory it sells and whom you can trust to "be around".