6 years of WoW experience here on multiple combinations of hardware.
I had a very very long post typed but I trashed it as I decided to go for simplicity.
You are at a lower res, which actually doesn't mean much for this particular game.
You're going to get great performance in Zones, 5/10 mans. I'm guessing in a 25 you'd average about 21-30 FPS. In a 5/10 man I would actually think you'd maintain 50+.
There are a few very important things to understand about WoW's hardware requirements.
1. When other players appear around you or you go to areas with people, this includes an instance, such as a 25man raid. WoW then becomes ABSURDLY cpu dependant. Mainly in cities/25's.
2. Enabling WoW's DX11 support on a DX11 card should give you an average performance increase of about 15-20 FPS, according to most benchmarks, as long as you're not using a DX11 card with an ancient processor etc.. I have first-hand experience with DX11's performance increase, it really is as great as it sounds. Blizzards goal was to increase performance, not graphics quality.
3. With any hardware from the past 4-5 years the most demanding setting of the game will be shadows, and this is extremely amplified with how many people you have around you.
Anyways, as I said above, you're going to get amazing performance compared to what you're used to. You can even get a 1080p display if you want..eventually. I would however go for more ram. And if you decide to go for that 4gb, make sure it's 2 sticks of 2 so you can dual channel it.
Edited by Ascii Aficionado - 8/21/11 at 4:14pm