Hi Mark,
Almost any modern instrument will be controllable by GPIB. You may want to check out www.ni.com/idnet and look for an instrument that has a driver written for it.
I might also suggest that you check out our DAQ cards. They can do analog I/O and are attached directly to the PCI bus making them much faster than communicating over GPIB.
Hope this helps out.
Best Regards,
Aaron K.
Application Engineer
National Instruments