Well, I have heard some people complain about Cataclysm being demanding, but I have an ancient HD2400 PRO with 2Gig Ram on Vista and it runs okay (30 FPS) on medium-low settings at 1024*768 for me.
So, I'd imagine you would be able to play on high settings at 1920*1080 without any problems. Just tweak the multi-sampling, draw distance or shadows to get the optimum balance of performance to eye candy as they make the biggest difference.
I'm thinking about returning to WoW to play Cata, but my PC is somewhat dated now and I'm just wondering how I could expect to run it.
E6750 2.66Ghz
2GB DDR2
4850 512MB
Will be running on a 24" Monitor at a rest of 1920X1080
Cheers