Hi,
i got the following warning while synthesizing in xilinx ISE 9.1.03.
WARNING:Xst:2254 - Area constraint could not be met for block <fft_ifft_serial>, final ratio is 298.
before the warning, the following lines appear.
Found area constraint ratio of 100 (+ 5) on block fft_ifft_serial, actual ratio is 303.
Optimizing block <fft_ifft_serial> to meet ratio 100 (+ 5) of 4656 slices :
The XST User Guide (I have 9.2i) has some words describing the area constraint ratio message in chapter "FPGA Optimization", section "Speed Optimization Under Area Constraint".