I would think that two 28W's would be fine. I was running a 400W 10K with two 15w atinics, and they would barely show through the 400 when it was on, but were decent for the dusk dawn effect. You'd have twice the fluorescent power and almost half the MH.

If you are trying for more color(blueish), I'd go all actinic, but if you will be viewing the tank alot during just PC lighting time, you'd probably want to go with a 50/50 bulb which is half atinic half daylight, or possibly 1 actinic and one 50/50...cs