Macbook pro 5.1 (late 2008?) RAM update - how much?

Hi there,

I'm very confused. Starting to feel the limit of 4gb on my MacBook pro, and interested in updating the RAM, but very confused about how much I can install, and the spec I need.

I am in the process of installing OSX Lion (so not sure if this updates any of the limitations I've read that older MacBook's have memory wise).

Here is my current spec:

Model Identifier: MacBookPro5,1

Processor Name: Intel Core 2 Duo

Processor Speed: 2.8 GHz

Number Of Processors: 1

Total Number Of Cores: 2

L2 Cache: 6 MB

Memory: 4 GB

Bus Speed: 1.07 GHz

Boot ROM Version: MBP51.007E.B05

SMC Version (system): 1.33f8

I'd sincerly appreciate input from a knowledgeable source! How much RAM can I/should I install, and how perhaps more details on the type of RAM and where to get it from *UK.

Many thanks!

Deep.