Radeon HD7750 is likely the best choice because its 28nm fabrication process allows it to provide better performance per watt than older GPUs that were fabricated at 40nm. Less power = less heat, so it can get by with a smaller cooler.
If you want to go more powerful (unlikely to be necessary) open the case and find out if you have any 6 or 8 pin connectors on your power supply. I attempted to give a mate my old 4850 but his Dell didn't have any connectors to power it...
Radeon HD7750 uses at most 55 watts, which is well within the maximum of 75 watts that can be drawn from the PCIe X16 slot, so no additional PCIe power connectors are required. However, if you find yourself in that situation again, you may be able to use a simple plug adapter (if the power supply has enough capacity on the +12V rails).http://www.newegg.com/Product/Product.a ... 6812887001http://www.newegg.com/Product/Product.a ... 6812189120http://www.newegg.com/Product/Product.a ... 6812267004http://www.newegg.com/Product/Product.a ... 6812200106
Many graphics cards will include these adapters in the box.
i7-6700K, NT06-Pro, GA-Z170N-Gaming5, 32 GiB, 980Ti, SM951, 5TB HDD, Blu-ray, FTZ01, SX600-G, 3007WFP+2001FP, RK-9000BR, MX518