I can't really back this up with hard figures but just from experience in my company and from talking with other geeks(!), Win2K is the best of the bunch from a power user point of view.
I wouldn't touch XP with a barge pole until they've upped it with a few more service packs. Win2K is pretty robust now and is a noticeable improvement on NT for personal use (don't forget to think about gaming and such-like that your bound to want to use your PC for!).
As to the version of Excel, there's nowt wrong with Excel2000 that I've found yet, although I bow to the experience of the rest of the board if they think different!
Finally, as a adjunct to this, if your spreadsheets are in the 10-100MB range, your much probably better off using a database instead.
If you've got a spreadsheet this big I'll bet that a great deal of it is just plain old rows and columns of data. If Excel is your preferred tool to work with then stick your data part in an Access/SQL/etc. database and you can still use Excel to query it, either by MSQuery or with the xlodbc.xla addin (all standard facets of the Excel install)
Rgds
AJ