In your processor's generation, the memory controllers were a part of the chipset (within the Memory Controller Hub (MCH) or the Graphics and Memory Controller Hub (GMCH), depending upon the chipset used) and were not within the processor. As a result, you see no maximum memory specification for this processor. Instead, the maximum amount of memory that can be supported was determined by the chipset used on the motherboard and the capabilities of the BIOS provided by this motherboard.
Without knowing exactly what chipset is being used with this processor, it is difficult for me to give you an accurate answer. In general, these chipsets supported 4GB or 8GB of DDR2 667/800 memory. If you can provide more information about your motherboard, I can give you a better answer.
Hope this helps,
The Chipset used is of Model I915GM Series with the 1.0 Version. And the BIOS is Phoenix AwardBIOS v6.00PG.
Wow, that's a few generations older than I expected (almost a waste of some of the capabilities of this processor). With the 915GM chipset, you are limited to 2GB of DDR2 400/533 memory.
But there is a DDR2 667 PC5300 RAM installed maybe because of that(old chipset) even the 32-bit OS works sluggish. Also if you can suggest a good motherboard/chipset for it, please do.
I hate to say it, but the best thing you could do is trash the whole mess and get something more modern. Even a modern Atom processor will kick this one's butt.