I'd recommend 2008 KOP wheels instead of plaction wheels if you are making 6 drivebases. $7 a wheel beats $29 when buying in bulk for an off-season project. They aren't shabby either, team 111 has used KOP wheels on their robots for years and still have one of the strongest drivebases.
http://www.andymark.com/product-p/am-0144.htm