I am wondering how much swap space I need in modern Linux such as FC6.
I have been thinking about that myself lately. I came up with the following reasoning: you need as much swap as necessary to fill the gap between your physical ram and the amount of memory needed for your most memory intensive task. So let's say you need 10GB of ram to crunch huge datasets. Since you have 8GB of physical RAM, you'd need a minimum of 2GB of swap. Swap is very slow. It obviously should not be considered as 'cheap ram'.