For DVD and other multimedia files, go with at least a 1GHz machine to be safe. You never know what future formats might require. As for video wise, go with a Radeon, they provide the best image quality, and the AIWs are great to have if you need a TV tuner too. If not, get a regular Radeon(7200/7500 at least). Sound card wise, skip the consumer stuff and go straight to stuff like the M-Audio Audiophile 2496, Delta 410 or Delta DiO 2496. They provide much better sound quality, whether digital or analog, and have much more features. Skip the Audiophile 2496 if you need to record stuff as it doesn't support recording, but if you don't record stuff, its great. If you must have TOSlink connectors, the Delta DiO 2496 is the only one to have it, the rest have coaxial connectors for their S/PDIF output. However, coaxial connections are better anyway.
As for DVD-ROM drives, basically almost anyone will do, but pay attention to things like the ability to make it region free, noise levels, and of course, whether you want tray loading or slot loading.
Memory wise, 256MB should suffice, but if you wanna play safe, you can add another 256MB when memory prices are cheaper.
Speaker selection is the hardest of all. If its gonna be a decent HT system I'd skip the computer speakers altogether, even the Promedias, and skip the HTiBs too, and get a nice set of small, cheap 5.1 speakers and a cheap DD/DTS 5.1 receiver as a starter, then work your way up from there. If you have a larger budget, then it make things easier.