http://secure.newegg.com/WishList/PublicWishDetail.aspx?WishListNumber=8376112
Honestly, the 780G chipset handles WoW pretty damn good on its own. But I set up this config with a 3650 with GDDR3 instead of the cheaper variants (tacks on a measily $10 compared the GDDR2). You'll get the benefit of hybrid CF, and power savings through the motherboard . If you're playing at 1024x768 or 1280x1024 you should be perfectly set for WoW(You really could not even use an add-in card, or even a **** 3450, and you'd have 40+ fps at most times).
I know it's over $300, by a fair margin, but if you start cuttings corners on things like the case, psu (how can you ask for any cheaper than what I suggested anyways) or the ram, you're entering dangerous territory. You'll be able to keep the case a good while, same with the psu without worrying of either crapping out(case door falling off, or plastic parts breaking--psu fan/etc. failing and killing your system).
I didn't see the 5000X2 BE in stock at newegg, and even if it were there, you'd have to get an add on fan because it doesn't come with one; so the 5400 should be fine--At least you won't need to muck around with any clock settings. I saw somebody picked out some $15 DDR2 667 2x1GB AR, but, I found some good OCZ stuff for $19 AR its DDR2 800 so it will be quicker than the previously mentioned, for a few bucks more.
I know the initial cost is going to hit you a bit harder, because it's nearly $400; but, if it matters to you, there's about $55 worth of rebates you can do to make the 'real' cost 350ish. You could get a 'similarly priced' Intel set-up, but there is absolutely no reason to overclock a cpu to play WoW when you could get a great AMD set-up. I don't mean to play favorites, but an AMD config will suit a WoW machine better (there's so much less work/money/headache involved) A few things to note; you could switch the case to something like the Cooler Master Centurion 5 which would give you the option to get something like a 3850 for graphics.
http://www.newegg.com/Product/Product.aspx?Item=N82E16811119068 -- Case
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102715 --Graphics