Check the free section of the Plans Now website ( http://www.plansnow.com/mdplans.html ). They have a few workbenches you can base your worktable on. You may also want to check this site for free plans .They have desk and workbenches plans. Some of them are pretty involved but the office desk ( http://ourhouse.ninemsn.com.au/ourhouse/fa...shop/08/896.asp ) seems fairly easy to realize.
For the table top, I recommend a technique I learned from the Shopnote magazine. You can use MDF as a body for your table top. It's a little heavier and more stable than plywood. To avoid the problem that MDF has of absorbing water, you glue a piece of 1/4" hardboard over the MDF. You can also use double-sided tape to be able to replace the hardboard. The hardboard is very resistant to pounding but not that resistant to cutting. Anyhow, it's a stable surface that is remarkebly smooth.