Well the first two recommendations are
1) If you are the HQ then look locally since you will want to "touch" someone (especially when things are going wrong) at one time or another and you want them as close as possible.
2) Start with inquires toyour current major suppliers of IT products. Most of the major players in the hardware market have packages and programs to handle needs as you describe. IBM, Compaq, Cisco, etc are all in the game, either directly or through agreements that provide a single point of contact with services tailored to your current environment. After all they do have a bit better understanding of what you have going now.
As to other names, well EDS come to mind, certainly IBM, Remedy Corporation (www.remedy.com/accelerate), and there are others.
It kind of depends on where the emphasis of your outsourcing plans are and how much you want or plan to retain in house.
Drop me a note if you want more information.
If the emphasis is on the help desk side there are a number of integrated with the web solutions, drop me a note and I will send them along to you.
