It may help if you can list out exactly what types of use you want the server to do. Depending on that... I may recommend VMWare ESX as the core OS and then you can run multiple different OS's under that and virtualize your infrastructure... makes life a lot easier.
Regarding Linux vs Windows.... that's really going to depend on what your needs actually are. A lot will argue that linux is the only sane way to go, however make no mistake that there are a TON of companies that rely on Windows everyday for a secure server and aren't disappointed. Also a lot of people think that Linux / Unix can't or won't be hacked... only to find out that they are quite wrong when it does happen. A poorly configured / not updated linux box will get hacked just about as quickly as a poorly configured / not updated Windows box.
Regarding the servers themselves, I really am liking the IBM x Series servers these days. Something like a new IBM 3650 M3 combined with VMWare ESX server could single handedly host probably 4 - 8 reasonably busy servers.... and if you're just looking for a really powerful SQL server... it's got that more than covered too with the ability to have 192 gb of RAM, and 8 Terabytes of storage, all in a 2U server that doesn't sound like a jet taking off ;-) In other words the thing scales great, has a bunch of features to keep it reliable, and it's not stupid loud like most are.