Here's an in-depth article which attempts to bring clarity to the "Dual-Channel vs. Single-Channel" debate. Hopefully it will help you determine the best RAM for your needs.

Final Conclusions quoted for ease of reference:

The Verdict: What Do These Results Mean; Is Dual-Channel "Worth It?"

Despite all that I thought I knew leading up to our MSI meeting last July, dual-channel just isn't necessary for the vast majority of the consumer market. Anyone doing serious simulation (CFD, parametric analysis) will heavily benefit from dual-channel configurations (~17.7% advantage). Users who push a lot of copy tasks through memory will also theoretically see benefits, depending on what software is controlling the tasking. Video editors and professionals will see noteworthy advantages in stream (RAM) previews and will see marginal advantages in render time. It is probably worth having in this instance -- in the very least, I'd always go dual-channel for editing / encoding if only for future advancements.

Gamers, mainstream users, and office users shouldn't care. Actually, at the end of the day, the same rule applies to everyone, simulation pro or not: It's density and frequency that matters, not channeling. Quad- and better channels theoretically have a more profound impact, but this is in-step with the increased density of kits that are targeted for quad-channel platforms. If you want to push speed, density and frequency should be at the top of your list. Generally, when you're spending that kind of money, you're going with a multi-channel kit of two or more anyway, but the point still stands...

As for whether it's "worth it" to get a kit of two, the answer is generally going to be yes -- but primarily because it's rare not to find a good deal with two sticks. If you're on a budget or an ultra-budget and are trying to spare every $5 or $10 you can, then perhaps grab a single stick of RAM. It feels so wrong saying that, but we have to trust the results of this test, and the results say that it simply doesn't matter for those types of users. Anyone building a ~$500 or cheaper system shouldn't spend the time of day being concerned about 2x4GB vs. 1x8GB as long as the price works out in their favor. Price is the biggest factor here, and with recent fluctuations, you're just going to have to check the market when you're buying.