Wow, for WoW you don't need all that horsepower. WoW uses a 7 year old graphics engine that has only received tweaks, but it's still the same as when it launched. Your 7750 is fine. My 6850 (@ stock) runs WoW @ 1920x1080 on Ultra at 60fps+. Only when I'm in Stormwind during peak times will it dip, but the lowest it's gone is 48fps.
If you play more graphics intensive games (Crysis, BF3, Skyrim, Metro 2033) then I would look at maybe upgrading, but not for world of warcraft.
When it comes to World of Warcraft's graphics, I look at it this way: You can put racing stripes and a spoiler on your turd but it's still a turd. Increasing the texture resolution slightly and adding a few polygons on a handful of models (and still reusing tons of old ones) does not a modern game make. It's 8th birthday is this November, and in 8 years it has not received a full graphics overhaul. It's not worth it at this stage anyway, it'd cost too much.
7750 should be ok. It's been said in this thread but I'll reiterate - ultra settings don't matter as much with this game because the engine and graphics are 8+ years old. What makes WoW's visuals distinct is the art design, not the polygon counts. That being said, a 7750 will be able to handle pretty much everything at 60+ fps, no need to upgrade if WoW is your only or main game.
You might experience some fps drops in 25 man raiding (not many people do that anyway though) or in a heavily populated city - but a lot of that is CPU related.
Simplexity88 brought up a point that I failed to mention, WoW is still very dependent on your CPU. Look at it this way, questing, farming & 5 mans are handled by the GPU. Once you get into the 10 mans, 25 mans or major cities during peak times, that's when your CPU starts to show it's true grit, so to speak.
The 7750 is plenty capable of handling WoW at High maybe even Ultra but also take a look at your CPU, which you didn't tell us.