another stick of RAM?

[Ogge]

1-Intel� Pentium� 4 CPU 2.66GHz, 2679MHz, 512KB (0% Load) .:. RAM usage: 257/512MB (50.20%) .:. GFX12 NVIDIA GeForce FX 5600,
Display 1024x768/32bit/60Hz Windows XP

This is what i have right now, im often fine except in huge zergs but sometimes when i gone go true a door i get 3-4sec of lag in randome places i also get 5-10sec of lag (i dunno if its lag screen dont do anything)