Hi.
I'm no expert either, but from the little I know, you will be saving energy.
2 400 watt hps. = .8 Kw/hr
24 26watt cfl - .624 Kw/hr

Your saving about 1.75 Kw/hr. If you have your last bill handy, it should say the cost of a Kw/hr. So, you can find out just how much your saving.
It might be kWh or something. I mean a kilowatt per hour rater anyways.

I grow 7 plants plants in a 40" X 40" space like this. Each lamp has 2 36", 36 watt lights. One cool white and one warm white. That's only about .216 Kw/hr.
And I use 1 400 watt hps in another space the same size to flower them. I only flower 3 now, but the next will try to fit 5, but at least 4 plants in the space. It seems to be working, and the total consumption is less then your 2- 400 watt lamps.

Another thing you can do to overall reduce the electricty useage is to replace every single light bulb in your house with those twister flourescent lights. (if you haven't already done so). Useing them just in your house will also offset the extra useage of your grow lights.

Cheers, hth.
BC.