for photo editing you will want plenty of ram and lots of storage so 512+ of ram and 120gig hard drive would be a good starting point. any modern pentium or athlon processor will do really but dont go for less than a 2gig p4 or athlon equivelent unless you have to. you will need a copy of windows so make sure its xp.
get a monitor that looks nice to you 19' crt or 17' tft at minimum. a seperate graphics card is not that important unless you wish to play modern games and onboard sound is fine if only using small desktop speakers but dont settle for less than 5.1
how much do you want to spend and what else is it going to be used for. spend as much as you can afford and get the highest spec you can now as it will last longer.