You could raise your memory manually to 1600 but the reason people put 1600 memory in a 1066 system is for overclocking. Using DDR 1600 lets you maintain the default memory clock of 8 as you raise BCLK all the way to 200.
133 BCLK x 20 CPU Mult = 2.66 GHz
133 BCLK x 8 Memory Mult = 1066 MHz
150 BCLK x 20 CPU Mult = 3.00
150 BCLK x 8 Memory Mult = 1200
167 BCLK x 20 CPU Mult = 3.33
167 BCLK x 8 Memory Mult = 1333
183 BCLK x 20 CPU Mult = 3.67
183 BCLK x 8 Memory Mult = 1467
200 BCLK x 20 CPU Mult = 4.00
200 BCLK x 8 Memory Mult = 1600