Yes. Your D40 uses a preflash. It's a good thing you asked. I see posts from time to time from people wondering why they can't trigger their slaves with their new digital camera, because most people don't realize that a preflash is being used. If you don't use a digital aware trigger, the slaves will trigger on the preflash versus the main flash, and won't contribute anything to the exposure.
Because of the differences in reflectivity between film and the CCD or CMOS sensors used in digital cameras, the vast majority of digital cameras (including your D40) are unable to meter during a flash exposure the way many film cameras worked (using off the film metering during a flash exposure to determine when to terminate the flash burst).
Instead, a digital camera like your D40 uses a very short preflash to help judge the length of the main flash burst needed (by measuring reflected light from the preflash first).
This preflash occurs approximately 100ms before the main flash, so most people don't even notice that it's happening (although a small percentage of the population will blink when they see it, resulting in photos with eyes partially shut or closed).
You'll need the digital version of the triggers if you want to go with an optical trigger using the camera's built in flash. Or, as an alternative, you may want to consider a wireless (non-optical) flash trigger instead.
That's a popular way to trigger slaves, and also has some added benefits. For example, you don't have to worry about a flash from someone elses camera triggering your slaves. ;-) But, this type of solution uses your hotshoe for the wireless transmitter (so your on camera flash would not contribute to the exposure if that's what you're trying to do).
I've seen some very inexpensive radio triggers. For example, http://www.gadgetinfinity.com
has some you may want to look at. They're a Hong Kong based site, but they have a very good reputation. They're a popular vendor for a little hotshoe adapter I use with a Konica Minolta Maxxum 5D, and they also list the same items on Ebay (check their feedback and you'll see many thousands of happy customers).
Or, if you want a higher quality system that offers more, look at the Pocket Wizard systems.
BTW, B&H sells the Digital Peanut triggers by the 3 pack at a reduced price if you prefer to go with optical triggers.