First could you make sure that your old DIMS are DDR3 or are they DDR2 ? Just checking, because even the board supports DDR3 your processor may not. Could you tell the specific model of your processor too please ?
Incase the old DIMS are DDR3 -> your cpu supports DDR3 then ->
I suggest that you plug only one of the new DDR3 dims in to the board and chech if the system boots up, if the systems boots, try adding the another dim too, if you cant boot after abother dim there can be 4GB memory limit by the board. If the system does not work even with only 1 dimm there could also be 2GB memory limitation for one memory line.
Your system does not really care if the dim is1600mz if your board / cpu cant run the memory at 1600mz the memory will be underclocked automaticaly by your bios.
Your new dims can run just fine at 1333 for example ( you can always run your memory underclocked no problem )
If you have the knowhow you can make sure that the memory is actualy running at 1333mz if you are able to boot your system with 1 or 2x new dims by entering the Bios ( if your board supports that ) and manually setting the memory speed to 1333mz ( not sure if your board supports this thou. )
There can be memory limitation of 4GB, but cant be sure about that.
The following information from the link you suplied.
( 4GB "limit" for 32-bit, does not mean that the board it self does not support more if running 64-bit os )
Dual channel memory architecture
Two DDR3 DIMM (240-pin) sockets
Supported DIMM types:
Non-ECC memory only, unbuffered
Supports 2GB DDR2 DIMMs
Supports up to 4 GB* on 32 bit PCs
*32-bit operating systems cannot address a full 4.0 GB of memory
Check if your old dims actualy are DDR3
If so try using only one new dimm ( only one ram stick installed ) and check if the system boots up.
If system boots up with one dimm add another and check if system boots up againg.
If system does not boot with 1 dimm there can be memory limitation per memory line of 2 GB
If the system does not boot with 2 dimms there can be memory limitation ( for bouth lines ) of 4 GB
You board / cpu / new DDR3 does not care if the dimms are 1666Mhz the memory speed will be undercloked to running at 1333Mhz ( DDR3 default ) and your new ram does not care about that and should work just fine at 1333 Mhz.
( i have mix ddr3 setup by my self )
1600Mhz Ripjaws 2x 8GB
Default voltage ( 1.5V )
2300Mhz ( cant remember brand ) 2 x 4GB
Default voltagr ( 1.8 V )
I am running them at 1750Mhz and voltage 1.59 V
And in between timeings and they work together just fine.
Sorry for messy text. I was tired and not good at writing text, but wanted to help.
Let me know if you got any results.