Trax,
There are many grades and species of pine. Of white pine in particular, I have noted it usually dries out too fast and warps, checks or cracks...especially near water. Martial Arts classes usually use it for 'board breaking' in under 10 year old testing demonstrations. Of the pine found at the local Lowes/HD I prefer to use Southern Yellow Pine or Douglas Fir.
To get the best possible inside dimensions for a 24x24 stand I would use 3/4" plywood. One sheet is all you need. If you can find Baltic Birch thats the best, but a Birch Hardwood Panel found at Lowes would give you a nice outside face for staining and be plenty of structural support. I would dado a shelf 4" off the bottom of the sides and rabbit the top in to the sides. Use Titebond II Glue on all joined areas. You can leave the back open or rabbit two panels into the sides that gives better support but still allows for cord access. These two panels can be as wide as you wish but generally I like to leave app. 8-10" opening between them for ventilation. HTH