I have a similar question of ClubMan, posted one year ago.
I've tried to update my PCG-K315 to 1Gb using 2 modules of 512 Mb each but the system continues viewing only 512 Mb total.
On Sony official website the RAM specs to upgrade are: 512 MB RAM PC2700 DDR SDRAM.
I was unable to buy the RAM from Sony official store, so I've found and bought 2 Corsair modules: 512 MB DDR SDRAM Corsair PC2700, 333MHz, CAS2.5, SO-DIMM.
The RAM supplier, after having get back and changed the first 2 modules supposing they were failed, now says it's due surely to the BIOS.
I've upgraded the BIOS to last version (Phoenix R0105X3) but still having the problem: system views only half of the RAM installed.
:mad:
Thank you for your help.

PS See attached link for official specs:
http://www.vaio-link.com/accessories/compat/compat.asp?l=it&m=1749&category=-1&serie=-1&c=10

Check the max ram the laptop can handle

As you can see on the link posted above (http://www.vaio-link.com/accessories/compat/compat.asp?l=it&m=1749&category=-1&serie=-1&c=10) the system can handle 1Gb.

Ok a couple of things to try.
Seems to me from what you have said is that one of your new modules is dead.
Try putting them in one at a time to check.
Either that or you are trying to add the wrong speed to your system as a mismatch can cause a similar problem.

Yeah srry about that the site wasn't in english OK hmm.. I'm not sure then but my last guess is some motherboard are picky on ram , I've known this from building a few computer
Try different brand maybe

The english version site is http://www.vaio-link.com/accessories/compat/compat.asp?l=en&m=1749&category=-1&serie=-1&c=10.
Remarks:
1. Tested one module a time: 256 Mb visible instead of 512Mb.
2. Modules are PC2700 (333Mhz). Sony web site suggests PC2700, crucial.com too, but Corsair suggests PC2100 (266Mhz). I've found on crucial.com that a DDR PC2700 memory is backward-compatible (see http://www.crucial.com/store/listparts.aspx?model=VAIO+PCG%2DK3x+Models) so it can work at PC2100 too. I wouldn't like to spend money again before understanding the right reason...
3. Sony e-support, after excluding RAM defects, thinks it's due to the motherboard and suggests me to repair it... but the PC is working well and I can't go without it right now.

Thanks.

I got a hold of the sony gods. I was having the same problem. This computer was way ahead of its time. I love this thing. But in its time its processor was compatible with 266mhz ramm speed. So the ramm upgrade can not exceed 266mhz. So the guy from sony said find 2x pc2100+ 512m at 266mhz. Blah blah blah. Do it. Indiekrush.com thanks.