The first step in all of this is to determine need. My company was having everyone in it use Macs for all of their work. This included not only all of Design and Editorial, but also Sales, Marketing, and anyone in Project Management. It became painfully obvious that approximately 40% of our company could be moved over to Windows-based systems for monetary savings, as well as an improvement to their overall ability to use more applications that were suited for the business world. For instance, as we started working on all of this, Palm announced that they would no longer be creating their software for the Mac, which was a definite minus against having people in the Sales department continue to use Macs. Also, ACT was not made for OS X at all and many sales people are rather fond of that program.
Naturally, Windows ain't for everyone and you still need Macs for the bulk of design work, but depending on your need, find Apple machines that make sense. From our tests, we have found little difference in how OS X.3 treats a dual processor machine than how it treats a single processor machine. While there are many tests out that claim massive amounts of speed from the dual machines, in reality, we simply have not seen it. This isn't to say that in future upgrades to the OS X software (such as X.4) that the benefits won't be seen, but it should also say that you can save yourself $500+ a machine by getting a single processor version if you're only looking for one to two years of expected lifespan.
The iMacs are a tempting choice, but honestly, the amount of money that you're paying for a flat LCD screen is not worth the extra $500+ that is required. We found that the eMac machines were actually quite reliable and good little performers. Taking into account how much less they were than the iMacs, it became an obvious choice to purchase these for people who did only Word processing and email, yet still required a Mac in order to proof materials from the designers that required font compatibility.
So, I recommend taking these things in to consideration. Keep in mind that you're most likely only going to get about three, maybe� four years of life from each of these machines, so spending top dollar on the absolute best machine makes little sense unless you have the money to burn and if you're in IT and do, then I have a digitally encrypted bridge I'd love to sell you.