My router is the US Robotics 8000 home router. And yes, it's got a 4-port hub built-in.

When the computers were connected with a crossover cable, they used the same NIC's (3Com Fast etherlink 10/100BAse-TX cards) as they are now. I also tried 3 different networking cables. The speed stayed the same. The network did work at 100Mbps, at least that's what I told win2000 to do. (I didn't leave the speed setting to "auto", I actually told win2000 to use 100Mbps/full duplex - didn't do much good for the speed though).
But ever since the router is on my network the speed has improved dramatically. Probably even doubled or more. Why this is so I can't explain.