I think you will get a lot of variation from that kind of question, people will obviously go by what they have found to be stable.
Personally, I would go for a 800mhz FSB Asus motherboard with a gig of ram. A p4 processor, probably a 2.6 will suffice and are relatively cheap now. Hard drive hmmm well I prefer maxtor but the new sata hard drives are damn fast but a little expensive still. The new Asus mobos work very well with these drives. If you are doing imaging then a good graphics card would be nice as you want the colour on the screen to be as close as possible to your scanner and printer.
A cd-writer to backup all your images is also a hot item on your list. If you think your hardware would suffer with winxp as in the way of driver availability, stay with 98se, but if drivers are available, go with XP as it is far more stable with new computer technology.
Also, go for a nice quality power supply, not a cheapo 200watts job. Good smooth power aids to a stable system.