Weather
November to February is the "cool season", which is dry and not too hot (up to about 30C or 85F). In April it gets really hot (40/100 daily, 30/85 at night), but not rainy. Starting around June it gets rainy--and still hot. It rains off and on all the time, so roads are muddy and some areas are impassable, and it stays like that until November, when cool & dry comes--gloriously--back.

Money
Cash is best. Bring US dollars if you already have them, or baht if you don't. US dollars (and to a lesser extent Thai baht) are accepted almost everywhere in Cambodia, intermingled freely with riel. You will get some riel as change when you spend dollars; just mix 'n' match. One dollar equals 3950 riel (as of April 2003); the riel has lost less than half its value since 1995 (those IMF policies keep inflation down, if nothing else). Coins are not in use.
There are a few places that will change travelers checks. Credit cards are useful only at a few ritzy places in Phnom Penh and Siem Reap, although you can get a cash advance from a Visa or JCB card at the Cambodian Commercial Bank, among others, in Phnom Penh and a few banks in other main towns.
There are no cash machines in Cambodia at all--before the Visa transfer started, residents used to hand-off their (foreign) cash cards to friends visiting Bangkok so the friends could withdraw money for them.  

Visa
As of 2003, Visas are available on arrival at the Phnom Penh and Siem Reap airports (see below), so if you are entering Cambodia at the airport, there's no need to get one beforehand. If you enter by land, you must get  your visa before you get there in most cases, and it must be marked for entry at that entry point. If it isn't, you are nearly certain to be sent back.

There is no other preparation needed that I can think of, except for a couple of shots, and for a short visit even those are probably not necessary. Havrix costs $60-100, but is thought to provide lifetime protection from hepatitis A, which is not a bad thing.