Front wheel could be any replacement of equal type but I'd first consider bumping up the tire size to 35mm then think of a replacement for the rear. How old are the old wheels. Are they worn, out of true or spokes broken?
"never want these wheels to go out of true"..it would help to say how much weight is being put on the wheels and if your previous wheels were taken to their limits in regular use. In other words did your old wheels not hold up over time, from a particular load or impact? Was it built well or did it go out of round from lack of maintenance?
If a particular impact took the rim out of round it may be a combination of rim strength but also inadequate tire size for the load. In other words you could have lots of spokes an little chance of the rim going out of round but if there's 350lbs sitting on that 32mm tire it may not be enough cushion to keep the rim flange from getting dinged.
Give PeterWhite a call. A 36spoke Mavic 719 on the front dyno should be as strong as what you had with a 40hole 719 on a 40 hole Velocity hub on the rear. But this is where knowing the weight matters, if you've got upwards of 350lbs total weight you probably need a beefier rim and bigger tires especially on the rear. A 27mm wide RynoLite may seem excessive but if your rear wheel load is at the upper end of the range you might as well go to the upper end of the range on rim and tire size for that frame.
I wouldn't bother with 4cross, it's the rim, spokes and build that matter.
Last edited by LeeG; 05-03-12 at 07:55 AM.