Some of you mentioned CPLDs/FPGAs. Well after a research I've done these days, I found that Altera, Xilinx and Lattice offer free VHDL code for implementing SDR SDRAM controllers. Only some changes have to be done to interface it to 8051 derivatives, because these VHDL designs are for 16bit/32bit CPUs/MCUs. Of course you 've got to be cautious with the timings. Also you can change the VHDL code, which the companies mentioned above offer, to support even DDR SDRAM...