I would recommend always buying memory from major brand names to ensure quality - but there are about 10 or so major brand names that all provide quality memory - some at reasonable prices. Crucial and Kingston are good names that generally have ram at good prices - especially their value ram. Their value rams are generally more than adequate for the average user. Other major brand names include: Corsair, Super Talent, Patriot, OCZ, Mushkin, Geil, and Samsung. Some have value, mainstream, and performance ram.
Pay the premium for performance ram only if you are building a premium system - with a fast CPU and or high end GPU - otherwise the features of the premium memory may be wasted.
However, consider picking memory that is compatible with motherboard - the MB manufacturer should have a list and many memory manufacturers have selection tools.
If you want to max speed - but not pay the full premium for high end ram, more important than manufacturer may be to pick the fastest ram your MB will handle while trying to get low memory timings. But consider only MB compatible memory and stick with major brands.
If you add, additional memory to your system, the system will use the speed of the slowest memory. So if you are adding 800Mhz memory to a system that already has 667Mhz memory - it will run at the slower speed. So it is generally best to match the slower memory (usually cheaper) or replace it all. I have on occassion added faster memory when I knew that later I would replace and upgrade the older memory.
Remember to match memory exactly (manufacturer, model, & size) if adding one unit to make a pair for dual channel memory.
The best news is that memory prices are EXTREMELY low right now so it is a good time to upgrade to 2GB or even 3GB of memory.