There are a few differences between the two.
1) The two-module kit is DDR2-667 memory (meaning it runs at 667Mhz effective), while the single module is DDR2-800 (runs at 800Mhz). The single module is faster.
2) Having two modules instead of one allows you to take advantage of dual channel. If you want 1GB, it is generally better to get two 512MB modules than a single 1Gb module.
So, which one to get, you ask? It depends. If you plan on doing FSB overclocking past 333Mhz (not unusual with the Conroes), get the single 1Gb 800Mhz module. If not, then I'd advice the dual channel kit. However, those modules have activity LEDs built-in. In my opinion, it's a waste of money, unless you're into flashy and useless things. You can probably shave off a few pounds by getting ram without LEDs.
Here's a 1GB 800Mhz dual channel kit: