THIS POST APPLICABLE TO MT (MINI TOWER) FORM FACTOR ONLY.
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 and one DDR3 4GB (GDDR5 is better than GDDR3, even if GDD3 offers 2 GB RAM or more), 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 DDR5 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. So older drivers are worse, can not comment about the newer ones, so this part is take it or leave it.
Next will follow maximum possible upgrade, 7750 is weaker...
7750 cards do not require power supply upgrade, 650 TI and 7770 are.
Unfortunately I bring you the bad news - not all cards can go inside Optiplex, and this is because Dell designed Optiplex for business not for home use. Inspiron in this case is much better in terms of upgrading.