From my personal experience, the GTX 550 Ti is easily able to handle 720p (2x Internal Resolution in Dolphin-speak) with 4x AA and 16x AF, and that's not even pushing it. It handles 4x IR, though I forget what AA I tried it on, didn't matter anyway because my LCD screen was too small. For AF, might as well go all the way to 16x. Pretty much any thoroughly modern GPU can handle it, even Intel's IGPs.
If you do get a new GPU, your CPU will then become your system's bottleneck. For your CPU, don't be surprised if you encounter a handful of games where you can't get full-speed, specifically certain demanding games or games that require LLE audio. Do try to OC like admin89 suggested, as that'll go a long way in Dolphin's performance.
If you do get a new GPU, your CPU will then become your system's bottleneck. For your CPU, don't be surprised if you encounter a handful of games where you can't get full-speed, specifically certain demanding games or games that require LLE audio. Do try to OC like admin89 suggested, as that'll go a long way in Dolphin's performance.