Wow is not all that graphically demanding. A 5770 could almost max it with AA off at that resolution. You shouldnt have any PC caused problems with a 5870 and an i5 750 CPU system.
Hard drives have almost no effect on FPS during gameplay, they only decrease load times when starting the game and changing maps. Somewhat the CPU and mostly GPU are what affects your FPS (other than server and internet issues).
yeah i thnk they up the graphics with every expansion.
I agree with Stinyweed, there have been improvements in the graphics since release 5 years ago but Blizzard still maintains a mainstream focus and as such have VERY low graphical requirements compared to most games. Crysis and the like are in a different class all together. I've looked over the WoW forums, as well as extreme building, maximum pc andvTom's and from what I can see your best bet it to get the fastest processor possible.
Most of the people that have very good FPS in the game run upwards of 3.6 GHz. They have one of the supported video cards on the WoW techincal forums. Also make sure you keep your shadows setting turned down, maxed it will kill almost any GPU. When you are in major town (Dalaran is currently the worst) make sure you turn down the view distance. The shear density of people will kill your FPS. In raids, again check your view distance and also turn down the setting for spell details. Doing all the things I mentioned will keep things running smooth.
If you are running 1920x1200, you will likely need to keep v-sync on as well as triple buffering, otherwise you may get some screen tearing. Also watch what addons your using, if you are a raider then no doubt you use addons. UI interfaces, boss mods and whatever else you've got running can/may reduce your FPS.