This is a decision process I went through a while back myself and it does, as other posters have said, depend on your needs and your budget. Without knowing more, we'll just be throwing our uninformed opinions around (but hey, that's what all these forums seem to be these days -shouting matches). For example, what kind of central server do you need? What are your mobile computing needs and what kind of uptime do you need? How much reliability, error compensation and capital outlay are you talking about and can you go cloud with your office server needs? Is environmental sustainability part of your office agenda, and if so, have you investigated your cooling and power usage for the server and individual workstations? Also, how many end-users are you talking about? Do you have a dedicated sales division which has different mobile computing needs?
For the server, obviously a laptop will not do as the cooling and temperature control would be crazy and it will not be moving around at all.
Desktops are certainly cheaper for any of the workstations which are permanent. Things which need to be left running for a long time or semi-continuously would be better as desktops for two reasons; efficiency and computing power. Most dedicated workstations fall into two types; basic permanent stations for clerical work and service terminals; and specialist processing terminals like for modelling and crunching.
For the first, a slim desktop PC is all you need - they don't take up much space, look good and can be connected to a variety of output devices and screens easily, they can run all day without having temperature control problems and can also be more energy efficient and better for the environment. You will not need much graphics ability or computing power for this sort of terminal. Additionally, for the specs you need to do everything you want on that terminal, it is much cheaper to go desktop. For the second, a desktop makes much more sense as you will need the best CPU/GPU setup with superior memory, great buses and very efficient cooling to get the maximum power out of the workstation, thus increasing efficiency by reducing wait-times for tasks. A laptop just doesn't deliver value in this category and it will always run slower and heat up faster. Also, it reduces your upgrade costs for these terminals down the line when you need to catch up with technology.
For everyone else, laptops will probably be fine. The sales division could probably make do with netbooks, like the Asus Eee Pc 1008ha/1005ha or HP mini 5101. Acer One and Toshiba also make good examples as do Lenovo and MSI. (dell's is less inspiring, forgive the pun). And Macs don't make one at all (this is true, please don't troll my statement, I'm not 'mac-bashing'). They're cheap ($250-400), super-mobile (half the size of normal laptops), perfectly suited to work-only applications by not having the power needed for games or the screens for watching movies, so they limit the staff to using them for professional application. If you can afford to wait, there will be a revolution in technology, die size reduction and energy efficiency within the next few months with a concordant reduction in price for the existing tech (which honestly is already enough). There will also be more integration with HSDPA and 3.5G usability integrated into these netbooks like Nokia and Asus are introducing with their latest releases so your mobile sales team is always connected to the office wherever there is mobile phone coverage.
As for PC vs Mac, there really isn't any need to talk about it these days. Macs can run Windows too so compatibility is not a problem, but it is only a very wealthy, image-conscious office which would need to go with Macs. For the same processing power, you can pay up to 25% more to go Mac, although their power consumption is lower and inter-office networking compatibility is higher. However, the Mac tech is deliberately very compartmentalised and your upgrade costs will be much much higher in the long run as Macs are not machines made to be at the cutting edge of processing power technology - they are made for device stability, internal compatibility and aesthetics; therefore you will eventually need to get a whole new set of computers for your office when the current setup gets 'obsoleted' by Apple. That said, I did consult for an office with a full mac setup a while ago and when things go wrong, they go very wrong and service techs cannot operate on Macs like they can on PCs. So instead of the usual 2-6 hour downtime, they had to wait for Mac techs to come and take the equipment away and fix it. Unless you have a specific need to be seen using a Mac, there probably isn't a reason to. If the individual staff member needs a Mac for personal reasons, then they should buy their own Mac for exactly that - personal use.
Hope this helps.