In my opinion you would be wasting your money unless your current machine is over 4 or 5 years old and seriously underpowered.

Your document structure is most likely why things are slow.
If you have over 100 section breaks you are asking Word to do far more work than should be necessary.
We've talked many times about footnotes and it sounds like you've stuck with putting large amounts of content into footnotes despite advice suggesting that life would be easier if you put that content into the body of the document.
The number of cross-refs are unlikely to be the reason things are slow apart from when you are updating those fields. Depending on your optional settings, fields could be updating when you open or print. However, if you have fields in your 100+ headers/footers then this could be a big drain on speed because repagination might be continuously asking these fields to update.

To test whether the speed problems you are having are really related to your hardware, make a copy of your complete 700 page doc and do these things to it.
1. Remove all Section breaks with a find and replace.
2. Convert footnotes to Endnotes
Now, try working with that file for a while and pay attention to the speed. Does it appear to work at an acceptable speed? If so, a hardware change is unlikely to fix your issues. However, if it is still slow, try locking all fields in the document - does the speed now increase? If not, try editing the doc in Draft view - how is the speed now?

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Andrew Lockton
Chrysalis Design, Melbourne Australia