Your probably gonna want a gigabit backbone if you're dealing with 500 clients...
Assuming you're running Win2k, Install the fastest server as a win2k Primary Domain Controller and install the other two as domain controllers in the same domain. Install the print queues on one of the domain controllers. Serving 50 printers is hard work so you're gonna need to get load balancing going to avoid stressing the print server...
since you're in one building, you can use Gigabit over copper. Get a 16 port managed gigabit switch, ten 24 port managed 100mbit switches with gigabit modules, and twenty 24 port 100Mbit switches. Arrange the 100mbit switches into ten groups of three, and cascade them so you end up effectively with ten 72 port switches, each with a gigabit module.
Put gigabit cards into the spare 66mhz/64bit PCI slots in you servers (The other 64/66 slot being taken up by the big f%$-off RAID controller) then run cables from each of the three servers to the gigabit switch, and from ten slots on the switch to the uplinks of the 72port switches. Make sure everything is set to full duplex, and you're away...
You'll end up with 720 ports floating around, and 550 things to plug into them (The printers of course having network cards.) This gives you plenty of flexibility with cabling. If money is tight, get rid of as many of the spare (module-less) 100mbit switches as possible.
If you want it really secure, you can use the management software to enable a MAC address based VLAN on the Switches - This will stop any unwanted machines being allowed into the network.