Well, then little bit of tech talk then.
Unlike CPU which is having a complex processing unit, a GPU or graphics processing Unit comprises of very simple but large number of processing units, called Stream processors. These stream processors perform all the 3D rendering job in parallel to each others, hence provide very high performance.
So more the number of Stream processors, more performance you can expect from the Graphics card (Keeping other constrains like memory speed, bus width same).
Now HD 5770 is having 800 Stream processors or 160, 5 dimensional stream processors (In ATI cards, 5 stream processors are coupled in a single unit, to process a single thread), whereas HD 5750 is having 720 or 144, 5 dimensional Stream processors. Other constrains like RAM (both are having 1 GB GDDR5 memory) and memory Bus (128 bit for both)
So ATI 5770 is better performer than HD 5750 due to its higher stream processor count.