Be aware that DVI is limited to 1920 x 1200 resolution. If your monitor is this resolution (or smaller), you need a DisplayPort to DVI adapter.
If you buy a large monitor with higher resolution than that, you'll need dual-link DVI instead. So you'll need the DisplayPort to dual-link DVI adapter.
Factor the adapter cost into your purchasing decision, as the dual-link adapter from Apple is spendy ($100). Also, DVI is not capable of sending audio, it only does video, so you'll need separate audio cables to use external speakers.
Your best option would be to buy a monitor that has a native DisplayPort connection. Then you won't need any video adapters at all. I think all the current Apple monitors have this.