Google says the maximum the motherboard in that computer will support is 1GB, at 512MB per slot. So yes, you do have to use the 2x512MB.
Intel Q9450 @ 3.2 Ghz
2x Sapphire HD4850 in Crossfire
4GB (2x2GB) OCZ Reaper DDR2 1066
Gigabyte GA-X48-DS4
Antec P182 Case
WD 160GB HDD + 2x WD 640GB HDD