I have to make a correction to my previous post.

The GX260 came in three different packages -- Small Form-Factor (SFF), Small Desktop, and Small Mini-Tower. It was the Small Desktop package that used the PCI riser, not the Small Form-Factor.

This eBay auction shows a picture of the motherboard with the extended slot and the riser card inserted:

http://www.ebay.com/itm/DELL-GX260-M...-/380268937983

These machines are probably 10-12 years old. I'm surprised that I remember anything about them...