You can run any bulb which is rated for the nominal wattage of an electronic ballast. In other words, a 250W e-ballast can run all 250W MH bulbs. The downside to an e-ballast is that it will generally produce less PAR than a comparable magnetic ballast with the same bulb.
With respect to the OP's original question of 175MH, the best combination for maximum PAR is a 175W electronic ballast with the Iwasaki 15,000K bulb. This combo will give you as much PAR as many 250W combos while using less electricity than a 150W HQI system.
It's all a balance between looks (which is subjective), PAR output and electrical usage.
The best place to look up actual objective measured PAR values along with actual electrical draw is Dr. Sanjay Joshi's lighting guide:
http://www.manhattanreefs.com/lighting
Just pick your bulb and/or ballast and it'll spit out all the information you'll need in the PPFD (PAR) and Watts columns.