Money is a complicated issue in Uzbekistan, due to the black market. The official rate of the Uzbek som is kept artificially high, and so everyone uses the black market, where your dollars get you approximately 30% more som. And it's easy to feel rich in Uzbekistan – the highest Uzbek note (1000S) is worth less than US$0.50 on the black market. One US$100 bill turns into a plastic bag full of ragged bills, usually tied together with a rubber band.
Bring US dollars with you to Uzbekistan. Euros can also be changed, but it's not as easy. Outside a few larger banks in Tashkent other currencies are totally useless.
A select few ATMs can be found in Tashkent, but you can't rely on them having cash in them. In the provinces, cash advances are usually possible at Asaka Bank for MasterCard holders and at the National Bank of Uzbekistan for Visa cardholders. Commission is 1% to 2% on MasterCard advances and 1% to 4% on Visa card advances. The NBU is also usually the best bet for cashing travellers cheques.