New DDR2/DDR3 compatible motherboards may be difficult to find.
ASRock G41M-S3 DDR3
ASRock G31M-S R2.0 DDR2
Your options are limited, but you may be able to find a quality used motherboard. Check what you can find from this list and verify CPU compatibility:
MSI X48C Platinum
Asus P5K-C P35
Asrock P43Twins 1600
ASROCK 775 G43 TWINS-FULLHD
If I were you I would upgrade both the motherboard and CPU.