1) An Internet merchant account. Most large banks offer these and they are usually about $10 per month. A normal merchant account won't work for this. This is a temporary holding account for the credit card payments that you receive.
2) An Online Payment processor. We use Cybersource for our business but we've also worked with Verisign as well. I'd imagine there are other companies that do this but these two are the big players. These businesses provide the mechanisms for your web site to do the actual real-time authorization and and money capture. The prices range between $15 and $50 per month (plus set up fees) depending on the level of sophistication of the authorization requests. i.e. The less expensive plans offer a pre-built form that lives on *their* servers and don't allow much customization while the more expensive plans let your programmer totally customize the web pages that process the cards, and these pages live on your web site. Once the card transaction has been approved these guys transfer the money into your Internet merchant account.
3) An Ecommerce System on your site. Your ecommerce system generally lives on your web site and handles the actual purchase process. It will do the talking to your Online Payment processor when it's time to charge the card. There are many, many ways to do this; from custom programmed code, to commercial packages, to free open source packages. None are easily implemented and all will have a set up cost associated with it. We've seen this range from $500 to $50,000. Your site designer will need to talk to you about all of the options, costs, and set up involved.
Bottom line: Real-time processing is great, but make sure that you are doing a fare amount of business via your web site before you go this route. Taking PayPal payments and manually approving card charges via your existing merchant terminal is much less expensive and a good way to get your feet wet.