The mobo in your Optiplex 755 has one PCIe 1.0A x16 slot:
http://support.dell.com/support/edocs/systems/op755/en/UG/HTML/sff_spec.htmWorthwhile, faster cards are going to be PCIe 2.0 (now being replaced by PCIe 3.0) cards. That poses two problems: (1) PCIE 2.0 cards are supposed to be backwrads compatible, but aren't always, and (2) at some point the speed of your graphics card may well be limited by the 50% slower PCIe slot you have.
Your PSU should be 275W. Dell PSUs are pretty good, ie, they are reasonable quality and deliver their rated power. So power might not be an issue.
You could perhaps put the mobo in a different case, but this is only practical if the mobo is mATX or ATX standard. Unfortunately, from these photos:
http://support.dell.com/support/edocs/systems/op755/en/UG/HTML/sysboard.htm#wp1131760the mobo appears to be proprietary, and based on the BTX standard. In other words, it won't fit in normal, commercially available cases.
Finally, there are likely some Dell proprietary connectors (eg, if you have 4 trouble lights on the case front panel) and other issues.
So the HD 6570 with the low profile adapter will fit in your case, it may run if its PCIE 1.0A compatible, and the power supply will likely have enough power to run it. If you can get return privileges, it might be worth a try.
Sorry, but that's the furthest I'd advise going in your situation.
