your best bet is to keep the pc you have and continue to run 2 wows through it then somehing like this

CPU= 95w so middle power rating tripple core unlocked multipliers should hit 3.2-3.4ghz on air
http://www.newegg.com/Product/Produc...82E16819103648 $99

Motherboard
http://www.newegg.com/Product/Produc...82E16813131376 $65
your decision here i prefer asus/dfi over ecs/msi/ pretty much all others

memory
http://www.newegg.com/Product/Produc...82E16820227267 x2=90$
im a g. skill memory fan myself but corsair make some good stuff and if they will fit the heatsinks are always a plus ( although i cannot testify to the effectiveness)

video card
http://www.newegg.com/Product/Produc...82E16814161276 $189 ( been on sale recently for as low as 160 plus the mail in rebate which i never count)
whereas you are correct in assuming crossfire can support dual monitors the 4670 is a severely underpowered card and wow does not support/fully utilize sli/crossfire setups, for the same price as 2 underefficient cards you can get the current non dual gpu "flagship" card which if you read reviews on is a monster, the reason for the lower price is they are currently clearing inventory for an late july refresh/new gpu debut)

power supply
http://www.newegg.com/Product/Produc...82E16817139005 $99
NEVER EVER EVER SKIMP ON POWER SUPPLY, off rbands are tempting and the numbers are impressive but stick to the big players here ( corsair,b antec better lines ( no green or value series), silverstone strider/olypus series etc) check out johny guru's power supply reviews for more ideas)

thats $455 a better system, more bang for buck and would run your remaining 3-4 wow's beautifully or all 5 if desired although you;d lag in dalaran etcud suggest spending what you can on "transferabel when you upgrade items" better hd/SSD(preffered)

i left out case because thats pure preference and since im oversea's im limited to local market unless i want to pay out the arse for shipping