out of memory error problem while simulating

HEllo

i am simulating a simple aperture fed antenna, but since i have increased the RAM from 1GB to 3GB, it does not get simulated and the simuation completes without results showing the following error

Fast sweep setup, process hf3d error: Matrix solver exception: out of memory, please verify Desired/Maximum RAM Limit setting..

Please help me out,, what should i do...