Depending on what kind of programs that will be used in their business use will help refine the parts selection.
For a basic system would start with an Asus P5B and a Core2Duo E4300 with 2 gigs of VS DDR2 667 ram.
For a video card and want DX10, something from the Nvidia 8400 or 8600 series. Or for more budget minded, the 7000 series Nvidea like an 7600GS or ATI X1600 or X1300 series.
From here we can build up to suit the needs of the system.
"When sliding down the banister of life; look out for splinters pointing up."