The Radeon HD 7750 has 79% the performance of the Radoen HD 7770. Another way of stating it is the Radeon HD 7770 is almost 27% more powerful on average than the Radeon HD 7750.
Click the link below to see the review and benchmarks at various resolutions. The review is almost one year but, it is a direct comparison of the two GPUs. The more powerful radeon HD 7790 was recently released and generally costs $140 (w/ rebate) to $160.