Neither the 730 GT nor the 1030 GT will make a beastly gaming computer. These two graphics cards are out performed by Ryzen integrated graphics and vastly outperformed by entry level gaming cards like the GTX 1050.
That Optiplex 3020 SFF also uses a proprietary, specially sized power supply so you cannot simply replace it with an ATX one. The PSU in that PC if I remember looks more like a long rectangle than a cube. If you had a non-SFF Optiplex it would be a different story. The only power supplies I see for that unit are all 255W which isn't enough to run even a 1050 GTX.
I would just buy another 4th gen Intel motherboard, buy a new case, and build a new PC out of the:
- Dell CPU
- Dell RAM
- Dell drives
- New motherboard
- New PSU
- New graphics card
- New case
Dell computers are not designed for upgrades and the proprietary power supply and motherboard severely limit your options.