Definitely Georgetown and Central City. Do yourself a favor and find Charles Wells' books on Colorado 4x4 Trails, and wouldn't be a bad idea to hook up with some of the local 4x4 groups, whether an in-person group that has to have meetings at set times and places, or an informal like
Colorado4x4.org - also check out
TrailDamage.com (damage
from the trail, not
to the trail) for other ideas on what's available and the difficulty.