Here is the link to the manual for the Dell Optiplex GX260 SFF: Dell Optiplex GX260 Manual
You can use Socket 478, P-4, 400 or 533 FSB only, 512K cache or smaller processors. You cannot use 800 or higher FSB and Prescott [1M or larger cache] processors.
With a Powerleap Modified CPU your options are a bit more. Powerleap has an interposer/cpu combo PL-P4P that does work on the GX260.
You must use NORTHWOOD CPUS or POWERLEAP modified cpus
The fastest processor the GX260 can support is the Socket 478 P-4, 3.06GHz 533FSB HT.
Just an FYI - it's been quite a few years since these chips have been manufactured so it may be difficult to find a new one out there. Also, a quick check of dell.com shows the Inspiron 570MT (no Monitor) starts at $399.99. Check it out here
For the cost and the fact it ships in 24 hours, you might be better of looking at new desktops instead of dumping money into a 4-5 year old box.
Hope this helps!