42. Oops, wrong question.
I have crawled Win2kPro on a Celeron300 w/32MB ram(it worked, just SLOW). I have 128 on my Pro machine, and it runs fine for general productivity. I would suggest 256, especaially if you have none...256 is a sweet price point, and only takes one slot (leaving room for more. I have 384 in my K6-2/500 training box, running Win2k BackOffice (complete suite, 120 day eval edition from MS for $15, btw) and Linux, and runs beautiful. Beyond 512, and you will not see much improvement, unless you are a server, 3D rendering, CAD, or intense multimedia development.
my 1.5 cents (I hate taxes!)