My 3.5-year-old editing computer uses DDR400 (PC3200) memory in dual-channel setup. I want to upgrade from 1GB (256MB x 4) to 2GB (1024MB x 2). I'm seeing faster, newer memory selling for half the price of PC3200. Why? Can I, for example, install PC6400 memory and have it still work at 400 MHz?

Go to Kingston.com and find your machine/motherboard. It will list ALL memory types it can support. If the type is not on the list, it won't work. RAM is so inexpensive now, I only buy new from Kingston and skip any "store specials".

...and never buy no-name memory or just the cheapest you can find. Memory quality determines the system stability, if it's not the best quality you won't have a lot of fun when rendering (video editing and rendering stresses the system and ram extremely, you need the best you can get!)

DDR2 RAM is not backwards compatible with DDR RAM.

You can run the higher speed memory at slower speeds. Sometimes memory that is rated for a slower speed has better memory latency, which makes a small difference in performance (but not meaningful for video editing applications, where memory latency doesn't mean much).

2- It's possible to test your RAM for errors with programs like Memtest, StressPrime95. I wouldn't necessarily bother... your RAM will probably work. If it doesn't, then your computer will have really weird behaviour and random crashing in which case you'd test your computer and figure out its the RAM and then RMA it.

That being said, brands like Crucial and Kingston can be pretty cheap.

Micron is North Americas largest memory manufacturer.
The below is their online storefront:

http://www.crucial.com/

Go to the above website and use their System Scanner.
The odds are excellent it will in short order tell you what
memory is compatible with your system and what Micron
has available and at what price.

Also, for what it is worth. After I install new memory, or a new hard drive for that matter, I always 'burn it in' by running test software such memtest on it overnight. It is simply cheap insurance and if there is a problem, you identify it quickly for handling the return/exchange with the vendor.

My 2 cents.

ya ram is really inexpensive, like just now bought a 1 GB for 30 bucks, go for the upgrade, but make sure your motherboard can handle 2 GB