Ram; You don't need a lot for this. Excess ram isn't gonna help. 64megs is probably a good amount (a guess).
Hard Drive; If you convert 1 VOB at a time you could possibly do it on a 2gig HD, but I would recommend at least a 10gig. If you need a new one get a 40-45 for about the same price. Speed shouldn't be a factor, but DMA will.
CPU; The biggest factor -speed will get you speed here!
DVD drive; Any should do fine, extraction can be sped up with a faster drive.
Video card; Not sure here, but you probably need at least a 4meg video card.
Sound card; For AC3 sound (DVD sound) you will need a sound card capable of 48000HZ or something like that. Although you could just convert to 44100 without playing the DVD at all.