Dougx1317 :
I've done some testing and here are the results:
Slot 1....Slot 2....Slot 3....Slot 4....Result
Correct Slots:
-----.......2GB.......-----.......2GB.......Does NOT work, and post beeps indicate "power error"
If you purchased the RAM individually then RMA {return} the RAM for a Matched Set. Further, if the RAM is NOT compatible then you'll get all sorts of oddball problems. Next, if the DIMM slots are damaged then time for a new MOBO.
Q - What Exact RAM do you have? {provide a link}
Also, see the Resource Monitor image below - the key numbers are the Hardware Reserved, Total & Installed. If the Hardware Reserved is above ~20MB {not GB} then you probably have a BIOS or Compatibility issue. You MOBO does not have an onboard GPU that might suck-up 0.25~0.75GB of physical RAM in the form of Hardware Reserved.
For the 'others' here, assuming no compatibly, onboard GPU, Partitions, bad DIMM slots, bent CPU pins, etc - 9/10 it's a DRAM Voltage issue and/or combined with a QPI/DRAM {IMC} Voltage issue(s). Set the DRAM at or slightly above spec IF the DRAM Frequency is set to RATED SPEED. By Default the RAM should run at 1.5v @ 1066 MHz {most systems} per JEDEC specs {SPD}.
Keep in mind it is VERY COMMON with P55 to have bent CPU pins, especially if there's missing RAM and exponentially so with Gigabyte MOBOs. This is a possibility with the OP. The OP might want to clean the DIMM Slots: Isopropyl alcohol - dampen the RAM pins, insert in DIMM slots, clean and repeat, and allow to dry ~5 minutes.
P55 Slots:
Windows 7 Resource Monitor: