Hi Freqmax,
As Alexh said, schmidtloads of RAM is good.
You didn't mention what manufacturer's chips you were planning to target?
As an example, this page on Altera's site :
http://www.altera.com/literature/rn/rn_qts_dev_support.pdfWill give you an idea how much RAM is recommended to target particular chips.
Presumably, Xilinx would have some similar guideline for their chips.
Good luck with it!
Red