Eventually processing cards directly in addition to via PayPal will help to increase your website sales.
We've found that some people, who do not have PayPal accounts, get frustrated with the hoops that PayPal sometimes makes them jump through to pay with a Credit Card (vs with a PayPal account). Others may have had a bad experience with PayPal and refuse to use them (rare, but has happened). Then there are those that would rather trust their credit card info to PayPal than to you and will only use PayPal, even if you offer them the option to use Credit Cards.
So it can, ultimately, increase sales to offer both PayPal AND direct credit card processing. We recommend that, once you reach a level of at least 5 - 7 orders per week, you look at setting up direct credit card processing. Prior to that volume level the cost is usually more than it is worth.
To accept credit cards on your website will require a dedicated IP address, SSL certificate and credit card merchant account.