Others will be able to fill in the Windows Active Directory architecture notes, if that's what you need, let me fill in what I know of the network stuff.
First a few questions questions as to what you need:
1. Internet access for everyone?
2. If so, business class DSL/Cable, T1, leased line? Lots of options, you can probably stick with a standard business class Cable/DSL solution from whoever is local.
3. Typical office programs or something crazy like large data set analysis ( 100+ gigs ) or video editing? You can probably stick with standard stuff without sacrificing much unless you are doing the later.
You probably need a business class router ( a Cisco 800 series or something similar ), although I imagine you could get by with a small business class one from Linksys or something. Then you should get a 48 port switch ( unmanaged should be fine for what you have going, it will save on costs ). I say 48 port because if you wind up needing more than the 24, you'll have to get a whole new switch, as you'd be maxed out. You want to provision for 30% more than you currently have, as a rule, for growth. This is being optimistic in the current economy, but it's a good rule of thumb.
Now, as for reliability, I've seen routers and switches with uptimes ( times running without reboots/failures ) greater than 1 year, so depending on the class you get, you should be fine. However, if you want to plan for that, it can be done. Unless you want each computer to have two NICS ( unlikely ), you would connect even numbered computers to one switch, and odd numbered to another. You'd then connect both switches to each other and each back to the router, giving you a somewhat distributed architecture. Now for your servers, you could definitely get 2 nics, and connect each to a different switch, and so on and so forth, for very good reliability. However, I myself would feel comfortable putting everybody on one switch and one router until the cost of another set was less than the cost of business lost if they went down.
It really depends on the type of business you are running. Amazon would loss millions if their site became unavailable for a few minutes, so they throw millions into distributed architectures and such. If you are more of a "Well, no internet today guys" kinda business, you could get by just fine with a couple business class devices. The more info you can provide, the better we ( the collective OC Forums intelligence we ) can get you info you can use.
Sorry if that's a bit of a rambling post, I've had a few beers....
-Drew
