I like the "Digital Greenhouse" ballasts I have from HTG, I have two 600 watt of these digitals running two cool tubes.
replace xx with tt...
hXxp://htgsupply.com/growlighttypes.asp?categoryID=1&subcategoryID=101
These ballasts are completely silent, and I was able to get two of them and upgrade the hoods to cool tubes for a total of just UNDER $500 to get everything to my front door in a plain box....I couldn't find a better deal anywhere.
Having used the ballasts for a few months I love them. They make no noise, have worked perfectly so far for over 6 months, no issues.
If I had the money I would invest in some of the new high powered tri-band LED's. A 600 watt costs $1,500-$2,500, but they claim you can get the same plant growth from one of these at 600 watts, than from two 1,000 watt HPS lights. The LED lights last ten years and use so much less power that they pay forthemselves over time, but who has the extra $1,500-$2,500 to invest in a light? Best part, those LED's use no ballast, create emuch less heat leaving almost no thermal footprint.
When i was looking at ballasts the only thing i really cared about was reliability. In my opinion the bulb you use has much more impact that the ballast so that is where I wanted to invest the money in the light set-up.