You cannot use DDR-3 -- it won't even fit into the sockets on the board (it's not compatible with DDR-2).

You can upgrade to as much as 8G of DDR-2 memory -- but bear in mind the memory is more expensive than DDR-3 and that it's dead-end -- no new system uses DDR-2 and hasn't for quite a while).

You need a 64-bit OS to use more than 8G of RAM, but XP64 is a particularly poor choice, since it was never widely supported with drivers (and 32-bit XP drivers won't work) and is an end of life OS now (No further security updates).

You could install a larger hard drive or a solid state drive - the latter being the single best hardware upgrade you can do.

The video is not upgradeable - and CPU upgrades make little sense (you already have one of the faster Core 2 Duo CPUs now).

Basically, the SSD is the only upgrade that makes much sense (though if you are still running XP, you should be upgrading that soon as well, for security reasons as above).

Hello enj63, Can I take advantage of the mode Dual Chanel Ram with 2 rams DDR-2. Is it supported? I've heard that we can run OS 64 bit with this mode. Thanks.

You can run dual-channel RAM, yes.  It's not required for a 64 bit OS - all that's required is a 64-bit CPU, which you do have.

Luckily, the CPU has 64bit-capable. I decide to add one more RAM and maybe also a SSD hard drive as well.

Thanks for quick reply and your support