What are the minimum requirements for internet bandwith and system to use services?
Any modern computer ( 5 y.o or less) connected to cable or ADSL internet (no matter what the speed of this internet connection) will work fine.
Even on Dial-Up connection it will work pretty fast, SAP systems do not exchange much traffic between GUI and SAP Backend.
Windows 7 system is new and not suported, but probably should work. Windows Vista, XP, 2000, etc will work fine, you need about 500 Mb space on your hard drive to install GUI.