There are a few pieces of information you need before you can figure out how powerful (expensive) of a graphics card you need.
1) What games are you playing- sounds like WoW and you want to multi-box. Are you playing on multiple monitors or just one?
2) What resolution do you play at? You can check this in the video settings in the options in just about any game. Usually you want to play at the highest resolution available for your monitor.
3) Do you intend to play with video settings turned up to high levels of detail? This is just a personal decision on whether or not spending more money on graphics hardware is worth it to you in order to have a better looking game experience.
Once you identify these answers it's a lot easier for people to help you w suggested video cards. Here are some basics:
Mid-range: HD 6850 or GTX 460 -- $150-180
High-end: HD 6950, HD 6970, GTX 560ti, GTX 570 -- $220-350
Really High-end: GTX 580, HD 6990, GTX 590 -- $500-600