The Desktop DOES NOT have a 305W power supply. You are Limited to 25W not 75W.
The riser does not change this. A radeon 6450 is 31Watts. Radeon 6570 is 60 Watts.
Using cards that take 75W can lead to power supply failure and total death of RAM, CPU, Hard Drive when the Power supply Fails overvoltage and Fries everything.
DELL™ OPTIPLEX™ 755 Specifications
Even if you get a Tower You CANNOT USE Double wide cards because they don't Physically fit.