The sites you see saying that DDR3 won't work on a DDR2 board is because they're talking about RAM, not the memory on a video card. You don't have to worry about the type of RAM on a video card really, anything should work so long as its a PCI-E 2.0 compatible card. Of course, better memory will give you better performance. But the card itself is the biggest factor in performance.
The important questions that need to be asked are:
- What's your budget?
- Besides WoW, what other games or programs will you be running?
- How much room do you have inside your case? Video cards keep getting longer and longer, and finding a video card that fits in your case is a real concern nowadays.
- How comfortable are you with overclocking?
- What is the rest of your computer like? No use buying a high-end video card if you're going to hit a CPU bottleneck
In terms of budget and performance needs, Tom's Hardware puts out an article once a month that suggests best buys for certain budgets. It also includes a performance hierarchy chart that I find invaluable. you can find the latest such article
here.
Of particular interest are the
Radeon HD 5770,
HD 5830, and the
GeForce GTX 460. These three cards all offer great performance-to-cost ratios. The HD 5770 is a good bit cheaper than the other two cards, but can be pretty easily overclocked to match or come close to the other two mentioned. If you're not comfortable with overclocking, the other two won't steer you wrong.
Of course, if you have more money to burn and want something blistering fast, you can take a look at cards like the HD 5850, 5870, or 5970. I'd recommend those over the other GeForce 4xx series cards, but the GeForce cards aren't too bad.
Another big factor as I mentioned above is your CPU and other computer specs. Let me know what those are and I can make further recommendations.