Unfortunately, according to specs, what you have is max for that PC.
Taken directly from manual for C521:
Memory
Type 533-MHz, 667-MHz, 800-MHz z (when available) DDR2 SDRAM
Memory connectors 4
Memory modules supported 256-MB, 512-MB, or 1-GB non-ECC
Minimum memory 256 MB
Maximum memory 4 GB