Yeah I'm in a similar situation, with my 780 SFF. I watched a YouTube video of a guy who put a Gigabyte GT730 2GB Low Profile GPU in his, and he installed it 100% for Minecraft. They go for around $70 - $80 on eBay. But I would like confirmation as well.

Yeah I have seen multiple videos like that on youtube as well. I have also been doing a lot of research to see what exactly it would cost to be able to get performance like we see in the videos. Seeing as to how you have a SFF i don't know how easy it would be for you to upgrade the PSU because before you upgrade anything on your Dell Optiplex 780 you are going to have to buy a new PSU so you can get enough wattage. The GPU for both of our types of computers only allows us to use low profile cards which will also limit our options on how well we can really make games perform and what cards we can get unless we get a new case. This list of components that I made below should all be able to run with just a 300w power supply and improve your gameplay "drastically".

CPU: Intel core 2 Quad Q9650 3.0GHz 

GPU: Gigabyte GT730 2GB Low Profile 

RAM/Memory: Now the Dell optiplex 780 is a little odd when it comes to RAM. The motherboards we have comes with four DIMM slots (where we put the RAM) and each DIMM slot only supports up to 2GB meaning we can have a max of 8GB. I have also seen a lot of people say you cant use DDR3 RAM on this pc, well, they"re wrong. It can use DDR3 just as long as it is DDR3 Synch DRAM Non-ECC Memory and clocks at 1066Mhz.