OK I got here on time, good. Mini Tower is not that "huge" as you think. You can not fit just any video card. Watch this video first, all Optiplexes share about the same hardware architecture http://www.youtube.com/watch?v=tbVoe-4_oZ0
Unfortunately no card that is more than 7.5 inches long or 2 slots wide will PHYSICALLY FIT inside the case - it was found that only single bracket video card can fit due to BTX MOBO, where card internals facing upward towards PC internals, not ATX, where dual card design is blocking another PCI slot by facing downward.
If you prefer the card with beefier fan, then the length of the card must not exceed 7.5 inches, otherwise only "skinny" single bracket, single slot cards can physically fit.
Pictures below will prove my point.
View shows no room for dual bracket
Single bracket only, or you have to cut dual bracket in half
Here you can clearly see only single place for the bracket
7.5 inches explained, also shown "obstacles"
This is the picture of actual "skinny" single slot video card (not sure which model)- perfect, "glove" fit.
So, I found 2 GDDR5 7750 cards (GDDR5 is better than GDDR3, even if GDD3 offers 2 GB RAM), which confirmed in reviews to fit Optiplex gx620, 745 and 755 cases (the difference between those models are different processors).
I cannot say which one of those 2 is better, both are good and got less bad reviews related to structural build. I saw recent recommendation regarding drivers "Changing to the just-released AMD catalyst 13.2 BETA driver improved performance noticeably in just about every game" posted 03/05/2013.
Next will follow maximum possible upgrade, 7750 is weaker...
Your choice is great for video editing and other tasks where quality of picture is more important than the speed of picture, if someone correct me it is ok, you choose DDR3 over DDR5, even with only 1 GB, DDR5 is faster than DDR3 with 4 GB. if the games are first priority. Read this review http://news.softpedia.com/news/PowerColor-Launches-Sing... so you see where is the "catch 22".