I bought 16gb ram for $400 but it is ddr2 and a slower speed 667mhz. You will have a hard time finding 12gb ram for a decent price.

I've 5 boxed on 4gb ram and got by everywhere except dalaran. You may be able to do it on 6gb.

Here is a link to 2gb ram chips for you computer for only $37.
http://www.bzboyz.com/ProductDetails.asp?ProdID=10733