Really, the high quality items are most important for boots/shoes, socks, and perhaps backpack.  After those, it is simply convenience (i.e., weight and size), preference, and style, of course.  I recently had excellent experiences in the Negev with my clothing consisting of quality Salomon hiking shoes and  2 pair of double-layer merino wool socks, and then I had 1 pair of zip-off pants (Gap $15), 1 long sleeve polyester athletic shirt (Walmart $6.50), a ball cap ($4), 2 pair of poly/nylon/spandex athletic undies (Walmart $7), and a lightweight windbreaking jacket w/hood (Gap $15).  Now, if you've got the $$ by all means go for the good stuff!  But, insofar as necessity is concerned you'll really just need to go lightweight, breathable, and durable.     

I agree with Seth's overview as well.

Generally, cotton shirts are considered good for hot hikes because it's good for the material to keep your sweat near your body. (That way, the same moisture will continue cooling and your body doesn't have to sweat more.) When hiking in the desert, I wore a regular cotton t-shirt and a loose linen long-sleeve shirt. Worked great. On the other hand, synthetic cloth that whisks sweat away from your skin - like polyester - is beneficial for colder weather hiking because you don't want that same moisture making you cold.

Zip off cargos are wonderful. The benefits are they're light-weight, quick drying (great for when you wash them, requiring only one pair), of course you get shorts and long pants in one, and you can unzip just the knee-vents if you want. Most any brand will work fine. I'd suggest something synthetic. I own some Magellan zip-offs that I bought at Academy for about $20. They're great; my only complaint (a small one) is that they didn't have a velcro or zip opening near the foot to widen the legs enough to take them off without taking my shoes off. Something to consider.

For a two-week trip, I wore one pair of synthetic underwear (easily washable, quick-drying), found in this thread. $25, but worked great.