That lag is usually texture loading thrashing the HD. You can reduce this by right clicking ww2.exe in windows explorer selecting Compatibility then NT. You need more ram since the footprint is aroudn 750meg and any less memory will hit the HD more often. Windows takes 120+ megs just to load and with anything at all running in the background 512 megs leaves very little to manage the sim.
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Get yourself 2 gigs of pc3200 - it will definatly help a lot