Hehe, a computer just to play WoW, am I seeing correctly?
300 is a bit low yes for a system, and 500 is probably alot more sustainable.
I doubt to play WoW you need that good of a graphics card, as long as you don't want the shader effects an an excellent resolution, you can do fine probably with an intergrated chip, though I wouldn't push it...
Though even a Geforce 5200 would do better than an intergrated, and this is a really cheap card, and would play WoW i'd imagine.
I wouldn't say you'd need that good of a system. Its just if this computer was going to be upgraded later on to new parts, where the problems would lie, as you wouldn't be able to afford a PCI-E motherboard, and room for a new CPU it seems.
500 though will definetly buy you a simple computer from somewhere like PC World, that will play that game. You many even get a rather good one for that price, but I wouldn't say 300 at all.
I hope this helps.