It should work, but at reduced frequency of 1333 MHz. That's not a problem.
You can't mix DDR3 and DDR2 so if you already have DDR2 memory installed, you can't install DDR3 unless you remove the DDR2 modules.
More info:
http://asrock.com/mb/Intel/G41C-GS/
Dual Channel DDR3/DDR2 memory technology
DDR3 and DDR2 are supported separately.
2 x DDR3 DIMM slots
Supports DDR3 1333(OC)/1066/800 non-ECC, un-buffered memory
Max. capacity of system memory: 8GB*