Ok, first of all WoW is a CPU/HDD limited game not GFX so above a certain point all cards perform equal, i.e right now in WOTLK you should be able to run the game on your 4870 with everything maxed out except maybe shadows (depending no your CPU). Ive just downgraded from a GTX260 to a 7900GTO while I wait for my new card to arrive and the difference is noticeable when I stop and gaze into the difference because the texture filtering is set lower but during a raid/instance its impossible to tell the difference during the action. Because I never keep a system for over 6 months ive run WoW on a lot of hardware and the biggest upgrade I ever saw was going from a 6800 GT to a 8800GTX, going from the 8800GTX to the GTX260 made no difference except I could max out the shadows, and that was only noticeable when stood still.
Now your probably thinking "wtf I asked about cataclysm" so ill address that, the are some improvements coming but nothing massive. The water is going from DX8.1 to DX9 and storm wind is getting higher res floor textures, the is more to it than that but nothing that warrants a Crysis style upgradefest.
My advice is to do nothing, wait until cata is released, all the new GFX features are not compulsory and can be turned down/off in settings to make it run like WOTLK if the new stuff causes trouble so you can get the game then see how much trouble it will cause and know exactly what kind of upgrade is required and upgrade accordingly.
IMO your CPU/ram are fine for cata no question I have the same ram and a Q9650 which your probably beating at your OC, combined with a GTX260 and a 15k SCSI drive the beta ran fine for me at 1920x1080 with everything maxed out but I believe that a lesser card would have done the job fine too.
Oh one last thing, WoW favours Nvidia, not as much as HAWX but its there