you're deadset on crossfiring then Newegg has a smoking deal on a Sapphire Radeon 6670 2gb
for $55 ($40 after rebate).
If you want a great discrete card at a great price Newegg has the MSI Radeon 7770 1gb
for $100 and you'll get a $30 pre paid card as a rebate, so it's actually $70 in the end. This will be faster than a crossfire hybrid.