would need to know what cpu and power supply, and everything else you have.

if you buy a gtx260, your cpu might become a bottle neck for the graphics card and you will still get low fps, that is if you have a last gen processor like pentium 4 or something like that. also I am not sure, but single channel ram can have a impact on stability and loading if you have a lot of windows open(i.e wow windows)

so you might be better of just limiting fps to 60 on main instance of wow and slave instaces you could lower that to 30fps, depending on the program you use to broadcast keys.

I would just save up for a better computer parts rather then buying 1 piece of hardware and hoping it will improve fps across the board. this way you can go all out and know you will have a decent computer rather then putting together a frankenstein computer :P