Understanding Transactions

I've been reading through the archive and I see that when doing a large
amount of inserts it is much faster to wrap a bunch of them in a
transaction. But here's my question. Say I need to do about 100,000
inserts and using COPY isn't an option. Is postgres going to do the
inserts faster in groups of 1,000 or 5,000? I know that letting each
insert be in its own transaction creates a lot of overhead, but I didn't
know if putting 5,000 inserts into a transaction created overhead for
that transaction. Hopefully my question makes sense. Thanks,

Joe