What makes AMD Trinity (and the older Llano) APUs good for games is it's ability to hybrid crossfire (or dual graphics) with another Radeon graphics card as long as it is not a power Radeon graphics card you are using. For Trinity the most powerful card you can use is the Radeon HD 6670 / HD 7670 (really a HD 6670 with a new name; it is not sold to consumer; only system builders). Installing a more powerful card or any nVidia graphics card will automatically disable the integrated graphics core. Also hybrid crossfire is only compatible with Radeon HD 6xxx and HD 7xxx series graphic cards.
Installing a Radeon HD 6670 will probably give performance equivalent to a Radeon HD 7750. Just a guess since I have not seen any benchmarks for a dual graphics solution.
As a standalone graphics solution, the Trinity A10-5800k has the most powerful integrated graphics core to date (I forget what's it's called... HD 7660G ???). However, it is less powerful than a Radeon HD 6570 desktop graphics card. It's good enough to play some games with medium graphics quality at 1366x768 resolution. The older the game the more likely it will perform better, I bet it will struggle a bit with Crysis and Crysis Warhead though.
Using any graphics card more powerful than the Radoen HD 6670 will defeat the advantage of having hybrid crossfire abilities. I think it is unlikely Radeon HD 8xxx cards will be supported (coming this summer I think).