Lots of thanks, Ilian
I think to buy the FPGA XC3S500E for to begin (your second link)
Can the devel of this FPGA to be make on a Linux box ?
(I think that yes, but in the doubt …)
And perhaps buy the Xilinx Spartan 6 LX75 FPGA USB Module 1.15b in a few months if I’m happy with the FPGA development 
The Xilinx ISE WebPack is available for linux, yes.
I have finally ordered a Xilinx FPGA Development Spartan-3E XC3S500E-PQG208 Board 4.3" TFT LCD 4 Nios
Where can I find one very basic/minimalistic “OpenGL (ES) like” HDL for this Spartan ?
(I think to work about multi-indexed v/t/n/c vertex reordering, texture/video compression and various transcendentales fonctions on it)
Note that I have found a french PDF that speak about transcendentals fonctions (and a lot of others things) on a FPGA at www.ens-lyon.fr/LIP/Pub/Rapports/PhD/PhD2007/PhD2007-04.pdf
And what is the free equivalent of Mapple on Linux that is the more adapted for to find minmax coefficients ?
(I have hear about Maxima, PariGp, GAP or Sage but don’t know them)