Depends on the infrared sensitivity of the D40. If it has good infrared sensitivity, you can shoot with an infrared filter, like the 87, 88A or 89B. But if you do use one of those filters, you won't be able to see anything through the viewfinder so you must compose with the camera on a tripod before screwing the filter on.

An option that I recommend is to have a point and shoot type camera converted to shoot infrared. I've been using a converted CoolPix 8400 with excellent results and am currently having a Canon SD870 converted. Here's a link to my CoolPix 8400 gallery:
http://bermangraphics.com/coolpix/8400-infrared-gallery.htm

Hi, I did allot of reading and questioning and got varied and quite different responses. So I took the leap and order a Hoya R72 filter $80. canadian. I used to take allot of ir images years ago with film and just loved it. I am happy to say, that yes, while I can't see through my dslr (nikon d40x), after a bit of experimenting I am loving it.it's cheaper than doing a conversion, and the results are awesome.

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I have long time admired i/r photos. years back with film i tried with some results, but can i do it without anything other than filters with my d40? or are there any tricks after downloading?
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Using filters is a tedious option that is fine for initial forays into infrared. But constantly screwing/unscewing the filter is a major headache, not to mention focus shift and focusing issues, increased noise, slow exposures, etc. And filter costs add up quickly if you need a variety of sizes.

To really enjoy IR, convert a camera for infrared use. This involves replacing the infrared-blocking glass over the sensor with infrared-passing (and visible light blocking) glass. I recommend glass that has a 50% cutoff at 715nm. Most cameras then allow exposures very similar to those of visible light, and of course there is no longer any need to spend money on filters, or deal with the hassles of using them. The ability to shoot at speeds like 1/250 @ f/11 makes a huge difference in how you use IR, so you're not restricted to static tripod-based shots.

The D40 will make an outstanding IR camera, but you could also start with any number of point and shoot models. See my Digital Infrared Photography Overview article for other tips.