overclock (not to be confused with overclockers) have a caveat in their terms
"2.4 If the vendor cannot supply the goods ordered by you, the vendor reserves the right to offer goods of equal or superior quality at no extra cost. If you do not wish to accept the alternative goods offered, you may cancel the order."
they still should have contacted you, call them, i'll bet they have no asrock g31 left, as its last years stock, no body is holding stock.
the gigabyte is vista compatible , nice! , for later upgrade
maybe win7 at a later date.
whatever dont worry check your proof of delivery check there is a guarantee ie 12 month warranty, if so dont worry.
If your still concerned ring CAB, ask about statutory rights.
btw asrock is a brand owned by Asustek
i personally use a few (mid price £40-50) of them, not had any issues, ok for desktop office use, asrock are moving into the upper end of the mobo market, it remains to be seen what the quality/price is, but i wont be buying £200 mobos total waste of cash imo.