The rendering times for my 43 minutes of video clips tell much the same story. There was little difference between the jump from 512MB and 1GB when I stuck to the non-dual-channel arrangement. Effectively, the video clip was rendered in 35 minutes and two seconds (2,102 seconds) when it had 512MB to work with, and 34 minutes and 50 seconds (2,090 seconds) when I upped it to 1GB.
In real-life applications, more memory means better performance -- especially when it's dual-channel.
When I rearranged the memory into dual-channel configuration (memory sockets are color-coded by pairs so you can tell where to put them), that time dropped to 31 minutes and 45 seconds (1,905 seconds). That's more than a four-minute improvement. If you do several renders each day, several times each week, you're talking days of saved time within a year just by putting your memory modules where they really want to be.
How about a larger memory size? Well, when I dropped in the third 512MB memory module, the render time actually slowed by 17 seconds to 32 minutes and 2 seconds (1,922 seconds). Why? That's right, class -- because I had upset dual-channel.
When I added the fourth 512MB module to balance the dual-channel arrangement, the render time dropped to 30 minutes and 31 seconds (1,831 seconds). The further decrease in time was the proof I needed to know for sure that the OSMark synthetic benchmark hadn't used the memory I added above 1GB.
Could I add even more memory and see a further improvement in rendering? The answer is probably yes. But, at that point, I'd begin to get into the law of diminishing returns. To begin with, since I only had four slots to play with, I'd need to toss out two of the memory modules I was using and replace them with higher values; for example, two 1GB modules. That's a major expense in purchasing the new memory and a loss of money on the old modules.
Unless you're seriously into video rendering (or any heavy-duty application), the upgrade probably wouldn't be worth the cost at the level of improvement you're likely to see. Why? Right now, the cost of a single 512MB memory module like the one I used is $105. (Memory prices are volatile, so the actual cost may vary.) The big improvement came from the switch to 1GB used in dual-channel mode, which doubled my original memory cost to $210. Doubling that cost again (to $420), to reach 2GB, would produce a much lower return on my investment.
If you go beyond the 2GB, you'll then be spending about 2.5 times more money -- remember, in order to reach 3GB, you're throwing away two of the original memory modules for a loss of $210, and adding two 1GB modules at a cost of $186 each -- a grand total of $1,192 overall for additional memory that would only produce another incremental improvement at best.
Unfortunately, while I've demonstrated that dual-channel is the preferred mode to use, I've also reinforced the idea that memory amounts are situational. It all depends on what you need the memory for. If you're rendering video for a living (or as part of a serious hobby), any time saved at each repetition of the task can begin to accrue enormously the more times you repeat that task. For most other situations, you've probably reached the value point for memory increases at 2GB. In fact, a serious argument could be made for just 1GB for the average hobbyist.
For video rendering, large spreadsheets, graphic image manipulation, and similar memory hogs, more memory is only better up to a point -- after which you might as well feed your hard-earned dollars into the paper shredder. Unless you don't mind throwing money at problems, you'll need to do some work to figure out when you've reached that tipping point.