From what I understand there is a great reseller there in the UK. I would suggest you contact him and discuss this with him. While my files are not as large as you need, I can open and work on files as large as 50meg and that is with 512ram and a 1.5g amd processor with on board video. I do know that opening Rhino and then again when opening a large file that is quite a spike in the processor usage and I have seen my ram max at 100%, but Rhino would still open and function.
http://www.rhinocad.co.uk should get you to the reseller.