This is an ad to my old post which recommended a build not a buy of a name brand.
In addition to my recommended parts I need to recommend a CPU. That depends on the nature of your business. But a Sempron or Celeron type cheapo should be sufficient.
I may also recommend a laptop. You can pick one up fairly cheap these days that are sufficient for many business tasks. On top of that you can opt for a 3 year warranty, as well your employee has the option of having the system on the go or at home which may benefit the business.
As far as Raid goes, if you absolutely need zero down time a mirrored RAID makes sense. Storage space may be required on this system as well, since a server or network storage of some sort for a small business is rare. It depends on the nature of the business.
As far as the guy recommending Dell or IBM if you have 40+ computers. Well if you have an IT guy in your business, and he actually knows what he's doing, I'd recommend a custom built system. Order up identical parts for 40+ systems, and build them all yourself. Way easier to repair than a Dell or IBM. Less likely to have problems. A Microsoft License may be purchased and your windows environment plus software may be configured to your liking, then a drive image can be created.
If you build the system properly you should have trouble free systems plus a 3 year + warranty. Abit has the best 3 year warranty on boards, Seagate 5 years on the hard drive, 3 years on the monitors, etc... The systems will also be loads faster than any Dell, IBM, or Compaq. No more employees complaining "I hate these f***en Dells they are crap and slow."
Try pricing up parts for 40+ systems. You'll get a deal. I sell computers for a living, and the suppliers will give a deal on 10+ identical parts. You also have some flexibility on the systems, you can alter the systems a bit to customize them to each employees needs if required. You can also upgrade the systems down the line. Keep all the cases and power supplies, jsut swap out the board, CPU and memory for 40 systems. Cheaper than renewing a 3 year lease on 40 systems. In the long run custom systems would be cheaper. ANd if money is of prime concern and that's it, you can also go Liux for your OS. Should be sufficient for business and if you know what your doing you can get one hell of a prime server running with it.
Also to end it off, don't hire an economics engineer or management type guy with a suit as your IT guy. Hire a ripped jeans and t-shirt wearing Computer gaming geek from your local LAN who knows how to do cabling, hardware, and internetworking. He'll do what's right, not f*&$ around with you telling you bullshit. You'll have the best business network in town built right to your budget. If this IT geek from the LAN your hire is really good, he'll get a business number, get set up with the suppliers, and start buying your systems wholesale instead of retail. He can also sell computers under the table on the side to benefit himself and the company at the same time. He can also set up a LAN, sell hardware there using his new venture, and have the business sponser it for a little free advertising and publicity.
Custum built clones, it's the only way to go.