GTX260 or a 4870 will blow two 4670s away.
The 4670 is a budget low-end gaming card for low power systems. Even if you put two of them together in Xfire, they will perform equal to or worse than a 4850. Xfire/SLi is more effective with higher end cards for better performance-cost ratio.
4670 ~ 8800GS/9600GSO < 3870 ~ 9600GT < 4830 ~ 8800/9800GT < 4850 ~ 9800GTX < 4870 ~ GTX260
Sell your 4670 and get yourself a single 4870 ($130 for 512mb or $150 for 1GB) or GTX260 ($130 for 192sp or $15o for $216sp)