I use an HP G72 laptop (i3/W7/64bit/4GB RAM) and Heinz 57 desktop (AMD Athlon/W7/32bit/3GB RAM). The laptop is used for tethered shooting, showing images after a portrait shoot and for storing images when I'm away from home. When at home the desktop is used for all image editing, storage and backups.
Both machines have ViewNX2, Capture NX2, Elements 9, LR3 and Photoshop CS5 Extended installed and both monitors are calibrated. I also have an external monitor I use when shooting portraits and at sports events.