Different Payment Gateway services offer different terms and conditions and impose different restrictions. Depending on your specific needs you might find that one service is much better for you than another.
In many cases, though, a site owner will simply want to get a web business off the ground and see some profits as quickly as possible. There's nothing wrong with that - it's perfectly okay to get started with one payment gateway and, as circumstances dictate, direct some or all sales through another service later.
For this purpose there is one service provider who stands out from the crowd: Paypal. Paypal allows you to open an account for free under your own name. Their basic service allows you to receive a very limited number of credit card payments - but they also offer another free account type, their Premier account, that allows you to receive an unlimited number of payments. These account types are ideal for somebody who is testing the waters or not expecting a high volume of transactions.
When do you need to start paying money for a Paypal account? If you need to trade under a company name Paypal will require that you open a business account. There are monthly fees associated with the business account types.
Any payment gateway service provider will hit you with transaction fees. Of course, Paypal is no different. However, if you are just starting your marketing efforts you will not be expecting a huge volume of transactions right away - so the slightly higher transaction fees charged by Paypal may not be a difficult burden to bear for the first little while that your new web store is operating.
Once you get started, though, you must be vigilant. Each payment gateway offers different types of services with different up-front and transaction-level fees. All payment gateways have rules and procedures that must be obeyed. Over time you will find that shopping around from time to time will highlight better deals when they become available. Again, this may not be an issue when the volume of transactions is low but it can be critical to your long-term success as the volume goes up or the margins go down.
See Also: E-Commerce Payment Gateways