According to the manufacturers site this is what memory can be used:
Support Dual Channel DDR2 533/667/800/1066(1066 by AM2+ CPU) MHz
2 x DDR2 DIMM Memory Slot
Max. Supports up to 8GB Memory
※It is recommended to use those Validated DDR2-1066 modules suggested by AMD
Taken from here
Would it be worth upgrading to 8GB of DDR2 RAM or should I go DDR3 and therefore have to buy a new motherboard?