go to run msconfig. click the boot tab ( Boot.ini for xp and older). under your correct os( in multi os systems) click the advanced options in the next box that pops up click the square that says maxmem and ajust also if you click on the #processer square you unlock multi thred and multi core processers
Before you can support something in software, it has to be supported in the hardware. Since his motherboard won't support more than 2 GB of ram, all the software changes you make won't let him support more than 2 GB. If the hardware can't 'see' it, haw can the software?