Mr Eusebi,
There are no specific requirements your computer needs to have with regards to the hardware you are purchasing (other than a PCI slot). If you are going to want to do extensive analysis while acquiring signals, it will be benificial to have a computer with a fast processor, but that is just a suggestion (not a requirement). See the document linked below for some additional information.
Minimum System Requirements Needed to Achieve DAQ Hardware Specifications
http://digital.ni.com/public.nsf/websearch/C0C96E1B55E459308025669F006E6334?OpenDocument