Okay, well before I can come up with a price, I'll run some things with you.
First of all, is the system custom built, or was it built by a manufacturer? If custom built, what type of case do you have, and is it in good condition?
What type of cooling is inside the system? In other words, is it standard air cooling, with fans, or is it water cooling? If it is fan cooled, how many fans do you have and what position(s) are they in?
Overall, what would you say the system has been the best with, performance wise? Have you stress tested the system with gaming, or did it perform better with media (video, music)?
What type of Optical drive(s) does the system include? These could be something like CD-R, CD-RW, DVD-RW, ect. - Also, for each drive, what is the speed of the drive (ex: 52X)?
Okay, now lets work with the information you provided...:
Processor Type: AMD 3500+ - Is this running at stock speed, or has it been overclocked before to run at a higher speed inside of BIOS? If you haven't set it yourself, it should not be overclocked.
RAM Module: 1GB of RAM is nice, but what type of RAM is it? RAM also has different reading times, but we may be able to target that down with the type of RAM it is.
Hard Drive: 200GB HDD - Is this SATA or IDE? What is the speed of the hard drive, as well as the manufacturer? Speeds could be read as 7200RPMs, for example. Some manufacturers would be Western Digital, or Seagate.
Video Card: The 6100 is a pretty weak video card based on times of today. I'd reccomend he do an upgrade if hes going to be playing extensive games.
Now, I require some more information:
Motherboard Type / Manufacturer - Any information you could provide on the motherboard would certainly help me to determine how reliable the motherboard is.
Age of the system - What is the estimated age of the system? Are any parts newer, due to upgrades, and if so, how old are they?
System Usage - How was the system used primarily before your attempt of selling it? Was it stressed constantly with games, or was it used as a word processor primarily?
System Condition - How was the system treated before your attempt of selling it? Was the inside of the system cleaned out occassionally to prevent dust build up and heat? What kind of area was the system stored in?
I apologize to overwhelm you with this, but I like to get as much information as possible before I provide anyone with an estimate on anything.