A 250W HPS isn't a huge amount of light for 18 square feet, but it is certainly a lot more than the fluorescents you have now. Very roughly, I'd guess about 3 times as much light. It would probably not be sufficient to light the whole table to grow your tomatoes through to flowering and fruiting, but perhaps enough to grow seedlings on until planting out. Position the HPS lamp, with a reflector, at a height which evenly illuminates the table, or that part of the table that has plants on it. 2-3 feet sounds about right.

Also, HPS light alone may not produce strong compact plants. Consider supplementing the HPS with blue fluorescent tubes to keep the plants growing well. With HPS plus 3 40W tubes, you might even be able to flower and fruit the plants.

Thanks for the help. It was what I needed to know.
I got the lights new for $50.00 each at an industrial surplus store. They are the kind that are used on the tall poles to light large parking lots. I had other plans for them when I bought them but if they would work on the plant tables (and it sounds like they will) I would them there.
I just need the lights until I can get the plants up, harden off and ready to go outside. I still have a month before it is safe to move them to the garden.