This guide is for MCDB graduate students and employees.
Prior to making your purchase, you should contact Jaime Birren or Jean Balch about how to fund your computer. In-coming faculty will have startup funds, some grants have limitations, and faculty get FCPP funds every fourth year, so let them help you figure things out. You might also want to see if Buff Surplus and Property Services has something that will satisfiy your needs (a great way to put a computer on some used lab instrumentation).
CU's OIT department has a good guide for those wishing to purchase a computer. You can follow the guide on your own if you wish. However it is highly advised that you consult with Dedicated Desktop Support (DDS) prior to making your purchase. If you are not familiar with DDS, go to their Prospective Customers page (see DDS link) and get yourself added to this excellent service.
When choosing your computer keep in mind that CU provides both cloud storage (5TB and up) and the PetaLibrary. Further, the PetaLibrary is directly connected to Research Computing's compute clusters. Try and take advantage of these services and save some money on your computer's local storage and CPU capability.
If you are planning on buying an Apple computer, take a look at MacRumors Buyers Guide. Sorry, there isn't anything so concise and useful for PCs, but your DDS support tech can help a lot with a Dell purchase.