How to cross the Thai-Cambodian border
First of all, beware of Thais trying to con you telling that the bus ride on the other side of Cambodia to Siem Reap will cost 600 baht and that it is impossible to get a ride once you have crossed the border. Just ignore them as well as people who want to 'help' you with the formalities and might try you to believe a visa will cost US$ 40.
The official fee is US$ 20 for a tourist visa (US$ 25 for a business visa). The Cambodian immigration officials will try to get more money e.g. by saying you can only pay in Thai baht, or that the price has gone up, or you have to pay extra for a 'health certificate'. It's all not true, but sometimes very difficult to get the official rate. A smile (and keep smiling!) will maybe help, as will just a few words of Khmer, like the polite way of saying "Hello": joom-riup-sooa. The price will probably go down, although maybe not to the official fee.
Once at the Cambodian side: do NOT change a lot of dollars into Cambodian currency, the riel. First, you don't need a lot of riel, as almost all prices are in dollars except for small purchases like a bottle of water. Second, the rate you get at the border is very bad: sometimes as low as 3000 riel to a dollar where the normal rate is around 4000 riel. If you want to change into riel, wait till you are in Siem Reap. The best rates and fastest service are usually offered by the money changers at markets.
Oh, and one more thing: Poipet is a very depressing place. You might think: is this Cambodia? Let's go back! Well, Poipet is about the worst place in Cambodia. Believe me, the rest of the country is much nicer and friendlier.
More tips on travelling in the Cambodia Travel Guide.