I'm not sure if you meant only GX260s and GX280s, or if the dash indicated some GX270s, because if it's the latter, make sure you give the motherboards a very good look. I can't remember the date range or service tag range offhand, but there's a range for both that pretty much 100% guaranteed that said GX270 was suspect to failing capacitors. Look it up online and I'm sure you'll find it. Even if it's not readily apparent, look at the ones hidden under the heatsink shroud, because I had a board (GX270 within the date range to fail) that had apparently already failed and had been "fixed" before I got it, and while it did "work", a few minor things (time, not detecting IDE drives once in a Blue moon, etc.) occurred because the few large capacitors under it were bulging a little, at least I suspect they were. Maybe they weren't, but even if it "worked", I'm sure something wonky was going on behind the scenes because those capacitors are probably needed, or they wouldn't be there. Even if there aren't any GX270s, both the GX260 and GX280 have also had the same problems, although a much lesser extent, so those should most likely be safe, but it's worth checking.I also have a stack of Dell Optiplex 260s - 280s that were replaced with newer computers at school that I could sell. Most in perfect shape, just needing a OS reinstall, and some need a new PSU. How much could these go for?
Edit: I found the date range in an article about it.
PCs plagued by bad capacitors - CNET News
Here's a forum thread that may prove helpful (shows the service tages that may be affected).