the degrees kelvin is the light's average colour temperature. 6700k or so is daylight above water's surface. 10000k is under water about 3m below the surface? 14k is a little deeper. 20000k is water 30m deep or so.
if you want to get picky on colour temperature, you can match according to research done on what you want/what you are keeping. Or just match according to your own taste. Advanced Aquarist has an article on how PAR is far more important than colour temperature. a 4000k 62 PAR light photosynthesized almost exactly the same as a 10000k 62 Par light. And keep in mind the human eye is most sensitive to yellow/reds, so when changing bulbs, don't wait till they look yellow. When you can see it as yellow, it means it's not outputting ANY blues, and it's PAR rating is now at half.
a rough generalization is that 10000k lights are twice the PAR as 20000k lights, but need to be supplemented by actinics to look "blue" like most people want their lights looking. You can take a cheaper route, and get 15000k's like XM 175/250w. Very good bang for the buck considering how much you'd save on actinics (the way I'm going). Just keep in mind, you'll have no "twilight" period where you can have just your actinics on in the morning/evening as a transition period. Prevents livestock stress.
And about depth, PC lighting is at 50% at 16" deep.
MH is point light, so it loses brightness in a radial way. 175w/150w MH is 50% at 20-24" away, and 250w is 50% at 24-28" away. putting 175w light over your 38 tank means anything at the top, 6" from the sides is getting 50% less light. Anything at the bottom 6" at the sides is getting 75% less light. You should save the money for two bulbs, or run PC's to supplement the single MH bulb, which may cost more than 2 MH lights if you decide to use a good flourescent ballast...
check out this site when comparing bulbs/ballasts. it's great for looking at PAR ratings, spectral curves, and comparing a specific bulb's performance on various ballasts.
http://www.reeflightinginfo.arvixe.com/
as for a reflector, you could use two 15" PFO parallel reflectors. They're nice because they hold PC lights as well, in case you decide more lighting later (which you will heh), but dont' want to change your whole MH system.
The shallow depth front to back of your tank though needs a smaller reflector. If you go with QHI double ended lights, you could gut a halogen workshop light for it's enclosure, reflector, and socket. They're designed for 500w halogen lights so they'll easily handle a little 150w HQI. I can't remember though if it's the 250w lights or the 150w lights you have to modify the socket slightly for (the distance between the 2 ends).
