According to your specs
http://support.dell.com/support/edocs/systems/dim3000/en/OM/j6758A04.pdf
Memory
Type 333- and 400-MHz DDR SDRAM
NOTE: DDR333 and DDR400 memory runs at 266MHz when
configured with Celeron 400MHz FSB processors
NOTE: DDR333 memory runs at 320MHz when configured with
Pentium 4 800MHz FSB processors
Memory connectors two
Memory capacities 128-, 256-, 512-, or 1-GB non-ECC
Minimum memory 128 MB
NOTE: Between 1 and 64 MB of system memory may be allocated to
support graphics, depending on system memory size and other
factors.
Maximum memory 2 GB
http://www.crucial.com/store/listparts.aspx?model=Dimension%203000&pl=Dell&cat=RAM