The speed is dictated by the slowest device that's communicating. If the router supports 1gig transfer rates and both pc's have gig nics installed, you'll be transferring at 1 gig.
Thanks! I linked all the PCs with 1 Gig nics to the router and then bought a 1 Gig switch to connect the downstream for my 100Mb printer and another 100Mb PC. As far as I can tell I have a 1 Gig network except for that one section.