Not strictly true - Athlon II X3s can potentially be unlocked to a quad core, and as I stated and CAT provided lots of lovely links to support, there is an AM2+ Athlon X2 5000+ @ 2.2GHz which is based on an AM2+ Phenom II and *may* be unlockable to a quad core.
In fact, any mobo with an SB710 / SB750 southbridge is theoretically capable of unlocking cores. if you're adamant about buying the Athlon X2 5000+ and you're keeping a discreet graphics card I'd personally go for a 760G motherboard like
this ASUS mobo from ebuyer for £43.90, however any mobo based on 760G or 785G should be OK - also there are a few 780G mobo that have SB710 but you would need to check carefully which southbridge the board had. As far as 790GX goes, there's only a couple of mATX 790GX's available:
this ASRock board from Ebuyer (£62.00) is probably your best bet if you can stretch the budget!
But where would be the fun in that