Hmm, that's surprising, my 6100 is essentially the same machine and also has 40 MB and is extremely slow, especially with Photoshop. Maybe Photoshop 6 (what I put on mine) is a lot heavier than 4?
For connecting to Ethernet, you should be able to select the Ethernet connection in the TCP/IP control panel and set it to DHCP. (You may have to install Open Transport first as it's OS 7)