Optiplex 990, video card upgrade

The best Dell validated low profile PCIe x16 video card for the Optiplex 990 DT was the =
6XMMP AMD Radeon HD6450, 1GB, DP/DVI-I

Or just check Newegg.

Well that's not even worth it :/ It's worse than my laptop's HD 5500

No, this not possible because this card is only available in full height form factor the desktop tower can only use low profile (half height) cards. Also, this uses two slots so either way, it won't fit inside the case.

this is the GTX 1050:

Should I get a 990 MT and get the 1050 or keep the DT and get a Low profile 1030?

The 990 Mt would have a similar form factor issue TWO SLOT CARDS DONT FIT IN THE CASE! I suggest you buy a dell precision t7500 as it will actually support the GTX 1050 you wanted in the first place

This might be an issue too:

The precision on the other hand:

Wouldn't that fit?

www.ebay.com/.../381848651525

It would if you don't mind loosing a PCI-E 1x since the tall shroud on the card technically would take a second slot the bracket appears to be single slot so it might just fit.

The 990 doesn't appear to have a combo riser or 270w power supply so no. 

2 SLOT wide cards FIT FINE IN THE Mini tower.  The short EVGA card would fit fine.  However 255w vs 305w power supply would be an issue.   The slot divider is taken into account.  Thats why even a Short Geforce 970 fits when you upgrade power supply.  Inspiron 3847 Tower has the same short slots.  Your advice is nonsense.

The PCI-E x1 slot in the way is also bogus.  2 slot wide cards cover that slot but it doesnt physically interfere.

Here is the same unit with GTX560 fitted and upgraded power supply.