If you do a raised bed, many will lay down several layers of wet newspaper or wet cardboard and then build the raised bed and fill it right over the grass, weeds, etc. The bed will be filled 6" or more (some recommend up to 12" or more raised bed). You should not need weed fabric under the bed, but may want to staple chicken wire to the bottom if you have problems with moles, etc. Read the FAQ's in this section for lots of info on what to fill the beds with and how to plant them using the square foot gardening method. You fill your bed with "Mel's Mix", a mixture of 1/3 each by volume coarse vermiculite, peat moss, and a mixture of 5 or more types/brands of compost. (keep in mind that bales of "compressed" peat moss approximately double in volume so read the bag that you are buying to see if it is compressed)

http://faq.gardenweb.com/faq/sqfoot/

FYI, a large amount of clover in your yard is a good indicator of a nitrogen deficiency. Apply some fertilizer and you should have fewer clover next year.

Or think of it as being 'green' since the clover is actually fixing nitrogen naturally.