Need some help replacing and finding the correct ram for a sony vaio!!

[Apokalipse]

So if i upgrade to 64 Bit OS then i will be able to use 2x4GB then? And it will definatly work?

As long as you get DDR2 SODIMM, and as long as there isn't some motherboard limitation. But I'd say it would be very rare for a DDR2 motherboard to not be able to use at least 8GB.

Sometimes a manufacturer will state there is a limitation, yet if you actually install more than the stated limitation it'll still work anyway.
I have a Sony Vaio YB laptop that I installed 8GB into, even though it says it only supports up to 4GB.

 

[Sheepykins]

well, you can install more, but a 32 bit operating system will only be able to use 3.2gb of any installed RAM.

Using a 64bit OS you can go much higher, but tbh I have 4gig in my laptop and its enough.
As apokalypse said, ANY pc2 series RAM will work, but pc2-6400 is the highest speed you can get in the PC2 range without overclocking.

you're looking for around 25 quid per 2gb stick. If you buy a RAM kit which usually has 2 sticks in, its slightly cheaper

Kingston DDR2 2GB 800MHz/PC2-6400 Laptop Memory Sodimm.. | Ebuyer.com
one kingston ram stick

Corsair 4GB (2x2GB) DDR2 800MHz/PC2-6400 Laptop Memory.. | Ebuyer.com
corsair 2x 2gb stick kit.