From my research, that laptop normally came with 4GB of DDR2 PC2-6400 (DDR2 800), Vista Home Premium 32bit.
Exactly WHICH operating system are you using? What does CPU-Z from cpuid.com show your system memory being as?
Below is information from Sony on that laptop model.
Memory Type: DDR2 PC2-6400, DDR2 (non-ECC)
Maximum Memory: 4GB
Each memory slot can hold DDR2 PC2-6400 with a maximum of 2GB per slot.*
*Not to exceed manufacturer supported memory.
Also, some system memory is shared with your GPU, and will not be available to your OS for use. You should consider using a 32bit platform with 4GB being the maximum the system supports.
I am not from Euro, (laptop was only available in europe apparently) so I am unsure of what websites are available to you where you can purchase RAM from if you do in-fact have 2GB of DDR2 800. Hopefully one of our euro users or someone that frequents them websites will come along and be of more help.
Also, please use the Edit Post button at the bottom of your post to insert additional information. Doubling up on posts doesn't get you help any faster.
EDIT: Maybe this will be of some help, after you determine exactly how much system RAM you have, it is best to use two sticks of the same type/brand/speed when upgrading RAM, which means you will want a 2*2GB kit, and nothing less, but I think your already running with 4GB...