As a rule Qliktech curerntly say the initial RAM footprint of a .qvw will be 10x the size of the .qvw on the HDD.
I find this to be a very rough guide but I guess it's the best we have to go one.
When calculating the amount of RAM that each new user will add Qliktech advise that this is 10% of the initial RAM footprint however I've found this to again be very rough for the reasons outlined in my blog post: http://qvdesign.wordpress.com/2012/01/30/set-analysis-good-for-you-bad-for-your-ram/
Your compression ratio should be directly related to the HDD > RAM ratio. This is determined by how efficient the column compression can be on your data - also you can alter the compression settings in the application but by default this is set to the highest setting I believe.
Hope that helps,
Matt - Visual Analytics Ltd
Qlikview Design Blog: http://QVDesign.wordpress.com
QlikView also employs a sophisticated data compression algorithm (as outlined in the QlikView Architecture and System Resource Usage Tech Brief) to allow for an extremely efficient usage of available RAM resources for data analysis. Compression ratios can range from 20% to 90%, depending on the nature of the data being compressed.
A general fact is: Low the cardinality in data, high the compression is.
CompressionRatio: range between 20%–90%
Hope this help,