Dual Channel memory reads Single channel

Hey guys, first post, been having this problem over the last few days... I bought some new dd2 ram recently (http://www.tigerdirect.com/applications/SearchTools/item-details.asp?Sku=P33-5074) its Patriot Dual Channel Viper 4096MB PC8500 DDR2 1066MHz Memory (2 x 2048MB) if you dont want to click the link. I set it to run in dual channel, one in the #2 slot and one in the #4 slot(because the cpu cooler is blocking the first slot due to the large 'viper' fins.) Anyway, i threw them in, started my comp up, everything is fine. A few weeks later i decided to OC my fsb 20 mhz, okay not a huge oc. I was in CPU Z and noticed that it read single channel(the Bios also said single channel), even though i had not touched the ram since it was first installed. Also, when i go into the system tab in windows, it reads 2.00 gb, instead of 2.75(im running 32 bit with sli graphics) But CPU Z reads all 4gb. Basically what im asking is why would my system stop recognizing dual channel if i OC my fbs, when i brought it down to 210 MHZ, it read dual channel again. Ive been searching all over for others with this problem and haven't found anything, please help if you can.

my system:
msi 570 sli ready k9 platinum
x2 evga 8800 GT
Patriot Dual Channel Viper 4096MB PC8500 DDR2 1066MHz Memory (2 x 2048MB)
AMD64 x2 4200+