I assume you prepaid the hotel already or have ensured the hotel will accept your credit card.

For food alone, RMB500/person should be more than enough for 36 hours, but it really depends on where and what you eat. Many places in China do not accept American Express, and i don't like using Visa when traveling (we were at a nice restaurant in Hangzhou that could only accept Visa with UnionPay logo). So cash is good.

I have never withdrawn RMB from ATM in PEK, but did that in ATM located in hotels in PVG. So I assume that should be OK. I would suggest you bring USD to exchange for RMB at the airport or in your hotel just in case. Use ATM if possible as the exchange rate is much better (but your bank may impose transaction fee).