Hi friends,
I think you can consider two solutions:
1- a short term solution: is what Mr. seanwu mentioned, but not in low level breaking down, i.e. flip-flops. I recommend you to break down or partiotion the design to 4 or 5 parts which can be synthesized without memory explosion.
2- a long term solution: you should upgrade your RAM and hopefully you can synthesize your design in a limited time.
I prefer the first solution, because the optimization will be done better than you have a large design.
Regards,
KH