To my knowledge the boot.ini file is mainly for choosing which operating system you want to start first if you are Dual Booting, I.e. having XP and Vista. Not sure how you'd set ram for games,

The game would use most of the ram anyway since it'd be your main process,

Oh and also, since you are using a 32 bit version of XP, you do realise that all your 4GB of RAM isn't being used by your OS? Windows XP x32 only recognises 3.2-ish GB of RAM - so you'd need to get a 64-bit OS to use all of it.