To database or not to database, that is the question...

I'm running Excel 2003 under XP. I have several Excel workbooks that
I've built up over time. They basically store sheets of data and have
various programs I've written in VB or DLL files that analyze and/or
plot data. Sometimes one Excel program opens several of these other
workbooks to access their data too. I'm the only user of this system.

All is well and they work as desired. The thing is that some are
getting rather large. The biggest one is nearly 100MB (and yes, I've
crunched the bloat out of it which Excel can create). It's basically
one sheet for each date, and each sheet is maybe 25 columns by 1800
rows. There are about 200 sheets at present and adding another each
week for just this one workbook. My only complaint is that it can
take time to open or save the large files. Otherwise they seem to
work ok.

My question is whether I'm tempting fate with such large Excel files.
Will Excel become cranky and unreliable at some point, or will it just
get slower as the files get bigger?

I don't feel it's worth recasting everything into a database
repository just for speed (which I could also improve with better
hardware), but I worry a bit about Excel reliability.

Thanks.

Bill