If space and venting aren't an issue, I'd definitely go with the 600. But I wouldn't use any computer power sources. I'm not an expert, but I think the main issue is that PSUs don't provide as much of a constant electrical flow. You need a negative resistance to the regulated positive flow, and I don't think a PSU does that.

$300 is pretty reasonable for that much light. I like Discount Hydroponics, and they've got a 600w for $189.95 that comes with a standard HPS bulb.

Discount Hydroponics - ValuLine Lighting

It's not a switchable ballast, though, if that's important to you. And shipping will be a lot. It seems like a couple of the places on ebay are Canadian-based companies...you might get a break on shipping there. Or, like I said, you can try Craig's List...my friend got a 400w light with an aircooled reflector for $100 on there. Just my .02.