It might depend on the latency of the ram you use. Some higher speed ram might have strange timings that the laptop doesn't like.

I just looked up your laptop model in the Crucial ram site and it states that it can use DDR2 PC2-5300 or DDR2 PC2-4200 SODIMMs.

Crucial have a 2GB ram kit for your model for £35.84 including VAT. You can look at the UK page for your system here. I've ordered ram directly from Crucial a number of times and it is good quality and a good price.

The EU and Spanish versions of the Crucial site have the 2GB kit for €31.71