Actually I would very much test out that other PC. There is no sense spending any money at all on an AGP graphics card unless you HAVE to and it seems as though you have access to a newer one.
While the x600 is not the "greatest" card so to speak, it will do a much better job then the current one AND you will more then likely notice a "cleaner" screen on some of your more closely worked on photos.
if you are somehow forced into using a new card for that AGP slot, i would say anything from nvidia that is 6600 or higher Back in the day the 6600 was the "gamers card" that could run significantly higher resolutions and keep great performance. I was not a dan of the ATI series from those days so I do not have a particulars from ATI for that level.
Also if possible use the DVI cable to hook up the monitor. You will not be disappointed.