First off, let me state that I'm an IT consultant who specializes in SBS, so I'm prejudiced.
SBS 2003 is a FANTASTIC product for companies with 2 to (roughly) 75 computers. SBS solves the vast majority of IT headaches that small businesses have.
The standard installation for SBS 2003 is to use two network cards, connecting one to the router and the other to a swtich. All client PCs are connected to the switch. This allows SBS to act as a firewall and avoids some common problems seen with SBS installations (DHCP and DNS configuration problems).
As a consultant, much of my business comes from fixing improperly-installed SBS setups. In a lot of ways, SBS is easier to use and manage than separate servers. But it IS an Exchange Server as well as a Domain Controller. And an ISA and SQL Server if you use SBS Premium. Vary from the built-in Wizards, and you are very likely to mess things up. The most common problems I see come from experienced IT people (often with NT or even Server 2000 or 2003 experience) attempting to outsmart the Wizards and do things manually. That's really a not-smart choice in the case of SBS. I'm an MCSE in Server 2003 and perfectly capable of manually configuring a DNS Server. But I NEVER touch an SBS's DNS Server except when repairing a messed-up installation done by somebody who ignored the Wizards.
Be VERY careful with joining client PCs and second Servers to the SBS Domain. Use the Wizards or you'll regret it! Also, be sure that you understand how to migrate current User profiles to their new SBS Domain profile. It's easy to end up with "lost" email or files. Also, pay REALLY close attention to your internal Domain name. DON'T use your "Internet Domain Name"!!!! And have a pre-designed strategy for User Names, Computer Names, and for your email Default Recipient Policy.
I get calls from a lot of new clients who've pre-purchased their Server, their hardware, and their software. Often, they've purchased hardware or software that's more costly than they needed and isn't required. $700 add-on backup software, for instance, isn't needed by many SBS owners. Also, some clients would be better served by backup solutions that don't involve tape. Many have purchased the wrong Antivirus software for their Server. If the clients had spent an hour with an SBS consultant BEFORE buying their Server, they would have saved a lot of money.
The convention in SBS-consultant-land is to do your first install of SBS THREE times. Once to make most of the mistakes, once to learn how to do it the right way, and once for the final install. I HIGHLY recommend reading one of the SBS 2003 textbooks first. Harry Brelsfords's beginning SBS 2003 book is a good first read.
If you are willing to do some reading first and have time to learn some details of Active Directory and Exchange, then it's certainly possible to install and manage SBS yourself. But there are some gotcha's:
1) If you get a Server with SBS pre-installed, consider re-installing it. The System Partition is usually too small and will likely cause you headache in the future. Also, you'll want to change the default location of User data folders and Exchange Stores.
Plan on spending significant time learning how to do the install correctly.
2) I recommend buying only RETAIL SBS 2003 and custom-installing it. It's about the same price and has much fewer licensing restrictions, and you'll get the correct partitioning of your drive(s).
3) If you don't do the SBS install correctly, you'll end up with a non-functional or problematic SBS Server that you may have to hire a consultant to fix. The fix will likely be a complete re-install, which will take considerably more time that it would to do it right the first time, because you now have important account information, data, and email to restore. I just did one of these two weeks ago. NOTHING worked right. None of the features worked. Remote access didn't work. The install was done by a part-time IT consultant with no SBS training or experience.
4) Without some professional instruction, you'll doubtless miss taking advantage of some of SBS's features, or will spend significant time finding them. It's sad to see companies ignoring the amazing abilities of Exchange, SharePoint, and Remote Web Workplace.
5) Decide in advance how you want to handle User account security. I recommend making all Users "Limited Users" on their computers and on the Domain. Doing this GREATLY reduces the chances of malware attacks but requires an understanding of how to work around problems caused by poorly-written software.
6) Require LONG passwords and enforce them with SBS policies. I recommend requiring 14-character, minimum, password lengths. Think: Easy-to-Use but Hard-to-Break PASSPHRASES.
7) What is your time worth? What is your business downtime worth? An experienced SBS consultant will do the install efficiently with near-zero interruption to your business. Trying to do this stuff yourself will take a lot of time and will, doubtless, interrupt your business.
Again, I'm prejudiced. But I'd recommend hiring an SBS-specific consultant to guide you through the installation. Microsoft Small Business Specialists have passed Microsoft SBS exams that test their ability to design, install and manage SBS 2003 networks. Microsoft maintains a listing of Small Business Specialists on its web site. A Specialist can help you set up your Internet (email and remote-access) connectivity (a common stumbling block), guide you through a safe backup and recovery strategy, and to tutor you on proper SBS Server management. And it'll give you an expert who already knows your Server setup to call on if you run into problems in the future.