Help with DT A2SD

I plan to move to GAOSP tommorow, so I put through an order for a 4GB SDHC Class 6 card. I don't want to mess with the internal memory of the phone (too much of a risk!), so I want to/will use an SD for cache and apps.

Do you have any specific instructions on how much cache is recommended, and how much space for the apps? Since I plan to use the internal memory for my 'casual' storage, I can dedicate the entire 4GB card to the system cache and apps. Any special instructions or advice?

I am particularly interested in the argument of EXT2 vs EXT3. Which one do you use and why? Most people tell me to go with EXT2, despite being a less versatile file system: EXT3 sometimes causes hangs?

Also can anyone tell me what 'swappy' is, what it means and what it does? I have been told to set it to '15'. What is that value exactly?