As much as I love Macs, I think I can be more productive writing code on a linux box. I'm going to assume that applies to everyone else :-), and say you should save some money on desktops by getting everyone a linux box with 2 Dell monitors.

Spend that money (and more) on getting everyone a MacBook Pro as well. As anonjon points out, if you don't give out laptops you might be discouraging working outside the office more than encouraging working at the office. An important part of startup culture, imho, is to not require face time at the office.

Shameless plug: I agree with you that encouraging collaboration is key. Since lots of teams are working in a distributed fashion these days, we think they need some other way to collaborate effectively. This is why we're building http://shoptalkapp.com . We've found that it even encourages more collaboration for those that are in the office.