new ram not being recognized, help!

[AllinDan]

Hi, I have a dell dimension xps 8300 with 512 mb ram stock. The system runs 2 256 mb dimms. I recently bought 2 new 1 gb dimms in the same type (pc 3200 400 mhz non ecc ddr) but only 512 mb of the new memory is recognized. If I put all 4 dimms in the system shows 1 gb, if I put just the 2 new dimms in the system shows 512 mb. I know the motherboard can handle up to 4 gb, and I tried reflashing the bios to the newest version, but to no avail.

what now?

thanks