[Memory] Gigabyte EX58-UD3R & DDR3 1600

Probably purchase another Operating System. If you are using a 32-bit Operating System (XP, Vista, or Win7), the OS cannot see and use more than about 3.5 GB of memory, which includes your RAM, GPU, and the CPU's L1 and L2 cache. The 64-bit versions will likely see and use all the RAM your board can see.

oops, forgot to mention I'm using 64bit Windows 7 and BIOS also only detects 2gb / 4gb ram.

The newegg reviews cite issues with defective ram slots and unserviceable BIOS chips. Based on the newegg reviews, I think you'd be better off RMAing and getting another make/model board. But if you want to keep it, I suggest you run memtest86+ on each of your sticks to confirm the RAM slots and RAM are working. I use the ISO version, which I burn as an image to a CD or USB stick. Then I set the first boot device to the media.

I have the same issue with the same board. A lot of people are finding it's bent pins on the motherboard. This seems all too common with this board. I doubt it's your RAM, but best to double check anyway.
I'm looking forward to getting a ASUS and trying for a RMA. So far Gigabyte have not offered me one. Very dissappointed and will never recommend Gigabyte.