But..
You have not said what the rest of your hardware is. In particular, the CPU will limit how much of your graphics card can be used.
For example:
The AMD X2-4800+ will be the bottleneck in about 5% of the games when paired with an HD3870. In most games the CPU runs at closer to 80%. The best balance would be an X2-4800+ and an HD4770.
You can look your CPU up on this chart:
http://www.cpubenchmark.net/index.php
The HD4650 512MB is a good choice for lower-end CPU's that are still powerful enough to use it. Don't get the 1GB version, it's marketing hype as a GPU at this level can't utilize 1GB for a single game on the market. You also should have at least 2GB of RAM if possible. 4GB is optimal and more than that is a waste unless you have a specific application need which is rare.
Here's an example of a lower end system that should play nice with WOW. I've looked at the chart for reference:
X2 3800+
HD4650 512MB
2GB DDR
This won't run a lot of the latest high-end games but WOW is one of the best optimized games around. The graphics also look best on a good CRT compared to any LCD monitor. You'd have to pry my 19" Viewsonic from my cold, dead hands. The viewing angle, contrast and color is still untouched by any LCD. I also watch movies on it which an LCD monitor can't compare with. Eventually I'll go OLED. If you can find a good used 19" CRT (hard to do) get one. You have to see it though.
About scaling:
Most people don't scale properly so they usually operate at a lower resolution so the text is large enough. There is no perfect solution but this is the best one. Do this for a 4:3 of 1600x1200 native res or widescreen of 1920x1200 (or 1920x1080).
1. Set to its highest resolution
2. Change DPI scaling to 150%
3. Disable DPI for programs that have issues with it (right-click the main EXE and click "Properties")
*DPI scaling should ONLY be used when your monitor is at its highest resolution. If you set a monitor to a lower resolution it DOES use all the pixels but it simply calculates the screen based on the lower resolution then scales it to fit the screen which doesn't produce pixels which are as sharp.
**Firefox presently does not work well with high DPI scaling (but Internet Explorer 8 works awesomely.)