I'm normally in the "you get what you pay for" crowd but some nice lights can be had for cheap.
www.htgsupply.com sells nice little setups for a good price. Many people use the htgsupply lights and their customer service is top notch. Not sure on the others but a nice 400 watt system (bulb, ballast, reflector) will run you about $150ish shipped. You can also get the kit that comes with a metal halide conversion lamp as well as the HPS lamp and still come in at under $200. I know a similar setup at my local hydro store will cost at least $250 without the MH bulb.
As far as wattage goes, it really depends on your grow situation. Obviously more watts is better but as you go up in watts the heat you have to deal with increases. I believe the 600 watters are the newest therefore most efficient as far as lumens per watt. You really have to take account of the square footage of your grow area and your ability to dissapate the heat that is generated.
Forgot to add, there are premium bulbs that have a few extra lumens and/or a little bit better light spectrum. They are no doubt nice but you can for sure get away with the plain jane bulb and do just fine. If you buy a kit that comes with a bulb I would just go ahead and use it and when it comes time to replace it you can buy the more expensive ones if you have the extra cheese $$.