The HD7750 is the slightly better of the 2. It would probably work with your current PSU to be perfectly honest. Neither the HD6670 nor the HD7750 are all that demanding in terms of power and Dell bundles pretty solid PSUs in their systems.
I use a HD6670 in a Shuttle XPC system with a 180w PSU just fine.
Just don't plan on overclocking the GPU and you'll be fine.
7750 will be faster. Although WOW has issues with playing at 60fps even on maxed out systems, due to the way the game works, it's not just the video card, CPU and internet connection and server load and amount of players in the area all have an effect.
You should be able to have a steady 40fps at medium though in any area.