If you have an online business where you accept orders and receive payments through your website, then your first priority (as far as payment methods to offer your clients) should be to accept Credit Cards & Debit Cards (Visa, MasterCard, AMEX, ect..), as most online shoppers will use that to pay for your products and services.
You should then focus on alternate payment methods to offer your customers, such as PayPal (make sure to use a Business account). Even though most people will use their Credit Cards, PayPal is becoming VERY popular among online buyers and should definitely be your #2 choice to accept payments (IMHO).
Then, you can take it further (as Conor mentioned) and accept Checks and Money Orders that are payable to your company's name. Although, be aware that these two methods are mostly preferred by local clients and businesses, and not many online buyers will use these.. So if you're not tapping into your local market just yet then I would just worry about accepting Credit Cards and PayPal.
Of course this will vary depending on the Company, Country, type of Business, Target Market, ect...