The answer is yes, your PC can handle 3 to 4 clients, 3 will be more comfortable than 4.

What is going to hold you back is actually the CPU, not your video card. You can drop graphics settings in WoW to lowest and get by just fine with most video cards that aren't low-end. My main PC uses an 8800 GTX, that's like 40 generations old now.

However, you can't really make the game use significantly less CPU. Ideally, you want 1 CPU core per instance of the game. If you have 2 instances on the same CPU core, that means each of them is basically limited to 50% of the CPU -- take your 2.66GHz and divide it by 2, and it'll be somewhat like playing 1 character on a 1.33GHz. It'll be slower, but manageable.

Anyway, I'd recommend filling out the 7-day trial application form and you can see how it will perform before making a decision