Precision 490 memory problem

I have a Precision 490 with 4 x 512MB sticks of memory. (in slots 1-4)

I purchased 2 x 2GB Mushkin memory from Newegg. http://www.newegg.com/Product/Product.aspx?Item=N82E16820226069

Based on the manual. I first tried leaving the 512MB sticks in 1-4 and put the two 2GB sticks in slots 5 and 6. The machine would fail to boot with "After POST" diagnostic codes 2 and 3 (which is a hard drive problem oddly enough)

I next tried just the two 2GB sticks in slots 1 and 2 with the rest of the memory removed. The system boots, but in the BIOS it says both sticks of memory are installed but the memory is running in single channel mode and only 2GB is being recognized.

I have tried many other slot combinations with either of the previous two results.

Can anyone offer me any advice?

Thanks,

Eric