It’d would be better if you move to a 64-bit OS rather than tweaking the .ini file.
By using XP Pro x64 or Vista X64, AE by default will us 4GB of ram and any extra ram will be kept of the system or any other app. And actually this is better since XP 32-bit can’t handle more than 4GB of ram at anytime.
Also in AE CS4 you can choose how many cores and memory will be dedicated for AE rendering, which is great in quad core configurations with +4GB of ram.