It's an ATX layout so there shouldn't be a problem. Measure the length available inside the case, and check the spec. on the card your looking at. IMO there's no real reason for 2 fans on a 75W GPU. If it needs a 6 pin PCIe cable then you need a new PSU. Again I see no need for the 6 pin cable or the bigger PSU at that level. A single fan card will work just fine.
Here are some build of that system.
http://www.userbenchmark.com/System/Dell-OptiPlex-790/3864