Dell Dimension 4600 memory question

My Dimension 4600 was running extremely slow so I decided to by more ram (I only had 256 MB).  After putting in two 1 GB sticks of ram I booted my system back up, it said only 256 MB of ram can be installed.  I checked the system BIOS (A07) and it would only allow me to go up to 256 MB max even though it said I had 2308 MB.

Am I supposed to update the BIOS in order for all the ram to be used? If so then which BIOS update?

in DIMM 1 and 2, I have two I put the 1 GB (PC3200)

in DIMM 3 and 4, I put the two old 128 MB(PC2700)