Vista was only 'yuck' for the first six months. Now it's as good as Leopard in most of the key ways, if not better. It's however not as self-absorbed-entitlist friendly perhaps.
As far as the server software is concerned, you can go with MS Small Business Server Standard, which gets you Server 2008 and Exchange 2007 - and is an affordable addition, especially when preloaded into a Dell server. If you want to stick with Microsoft for database, then there's the Small Business Server Premium edition, which ships with an additional copy of Server 2008 and SQL Server. You'll also need an addition 5-user CAL for either (10 users total), and an additional 5 when you expand.
These are powerful suites. Provided you know what's under the hood, what you get is essentially the same tools as large corporates are using at their core. But it requires appropriate knowledge to get the most out of it. As much as I like the idea of MS SBS, I don't think it actually works all that well in companies where you don't have someone who is able to configure / manage it properly. And I think that's the majority of very small businesses with no need for their server to talk to the outside world in any other way than mail.
If you also want to maintain your PC's, you'll also need systems management. There's many ways to go about this - but really, if you are a Mac user or even someone who would Switch given the chance, and therefore use/prefer a computer for the level of technological ignorance it panders to, it's better to get others to do it for you.
And there is a way of doing that, and also kicking Microsoft completely out of the equation as far as the server is concerned. If you're a basic outfit, as I said above this will not be an issue and you can retain Windows PC's for users. Take a look at the
Zybert Z1. It's an outfit run from academia, so they might also be able to help you out in more ways than a company like Dell. The server is based around common Linux technologies, tied in with a user-friendly shell. The rest is handled remotely via a low-cost support contract with Zybert. It's capabilities are low compared to SBS, but then it probably does all of what the typical office user* would think of doing.
*The sort of people who say to you "I'm not very good with computers" while having been sat in front of one for 8 hours a day for several years
