Last year I started a 10x20 bed.  I went overboard with the variety, but made out pretty well with cukes, tomatoes and herbs.  I started with regular yard soil (mostly clay) and tilled-in mulched oak leaves, lime and peat.  Overall, it was not a success.  I planted right into the ground, but the plants were not doing well, so I switched tactics - I dug up all the plants and re-planted them by digging a much bigger hole and filling with composted topsoil.  The plants seemed to do better, but I think my problem was that I used landscaping fabric.  My thinking is that it "baked" the roots systems, but I'm no expert.  I didn't replant this year because of MAJOR excavation in my yard (new septic), but next year I plan to rip out all the landscape fabric and plant in similar fashion - dig a large hole and fill with composted topsoil.