Well that all depends on your budget, I got away throughout most of my grow with a couple of T8 flouros from walmart @ only $10 a piece, if you can afford it though you can pick up a 100watt cfl (starting $40) all the way up to a 225watt I saw yesterday for about $139. The t8's run 64 watts between the pair[2 lamp fixture] and you can spread the light out better over your plants with flouros (t5, t8, t12). I fit one of these fixtures in a similar sized box; just pitch it at an angle and you should be good.
HPS= High pressure sodium (used in flowering)
MH= Metal Halide (used in veg period)
These two lamp types are Primo gear and will make you plants boom but they come with costs
the most notable being :
A: Substantial heat
B: Power Consumption
C: Initial Purchase Cost ( Check Craigslist or online b4 the hydro store)
And the benefits
A: Bigger plants through better light penetration/ greater lumen output( increased light delivery to potential bud sites)
B:Quick turn around ( veg in half the time of Flourescents)
A Metal Halide conversion lamp is a MH lamp that can be used in a HPS fixture/ Ballast (saves money starting out, eliminating the cost of two lighting units)
Flouros cost no more than $25 including lamps @ walmart and they will get you off to a great start.
Lights of America makes a great 2 light t8 fixture for $10 and then just get yourself a pair of Phillips daylight deluxe 4 ft t8 lamps for another $6.95 and a few more $$$ for the hardware.