Currency is definitely a cause of paranoia and worry for me when travelling.
I have a tendency when travelling to organise the trip myself, what do I mean?, I mean I like to book everything separately. I like to have full control of where I stay and how I get there and rarely if ever use the services of a travel agent. It is something that I enjoy doing and is all part of the planning ('p' post!) but it means that I am quite careful with my budget and have to allow for accommodation expense during the trip as it is not all paid upfront.
I usually end up with the currency for that country, some British pounds as back up, and also use a visa card. I am not great at estimating how much I will spend and it is something that hasn't really improved over time, but a good guesstimate (my mothers advice here!) is that you will spend as much as you do on travel and accommodation (or on the package deal if that's your route) on food and spending money. Although it does depend on the individual. Shoppers among you may spend more and if you are like me you wont spend anything until the day before you leave and realise you have no souvenirs and haven't eaten for 7 days!
I always have a purse with the days budget in it and other money is kept elsewhere and it works for me, some people find it useful to make a quick currency conversion card that they can have in order to work out what they are spending and it can be useful if you are in a country with an odd exchange rate.
When I went to America I decided to get some US dollars and also to take with me a paid card. This is like a credit card that you have put money on and then can pay for goods and services with the card. I think these are quite common in the US. It was a good idea except you cannot get a balance. So even though I could access ATM's and withdraw money and pay in shops and restaurants, I could not get a balance not even from inside a bank. I didn't discover this until the last day that I was there and wanted to spend the remaining money. I would advise that if you choose this option that you keep a tally of what you spend on it as I was not impressed, seriously stressed out and didn't buy what I wanted and had to just keep spending until the card was refused. I am still undecided if I would use it again, as a paranoid traveller (who is now discovering she may just be a control freak) I had no control over it.