Sounds your dealer doesn't exactly know what they are talking about. In most cases a PCI (which is 32-bit) 10/100Mbps NIC card will do. You are only as fast as your slowest part. If for some odd reason your other computers have older, ISA (16-bit) 10Mbps NICS then you will only communicate at 10Mbps, not 100Mbps.
Maybe they were refering to general Internet use?? Our 2 upstairs computers are connected by 10Mbps Linksys Phonlink NICS and are plenty fast for Internet use.
Behold the POWER of common sense!!!