IME unless you're doing an awful lot, 128M is fine even for Win2K. That's what I have at work, on a P-III 450, and it's fine. I've a dual-head setup and even with a couple of dozen apps (NOT windows in the same app) running it doesn't struggle. So the thought of needing even that much just for IE is, in my opinion, just plain daft. Sure, apps are getting more wasteful, but it's only a couple of years since 32M was adequate for 98.
OTOH memory is so cheap that if you're not changing your machine in the next couple of years, you might as well get it up to 256M because by then apps will be so wasteful you might actually need it. If you'll be changing your machine sooner, you'll probably be buying different memory (DDR or RAMBUS) so you wouldn't want to bother buying SDRAM now.