If the wheel's not too old, it may be worth using the original spokes, especially since it's a front wheel. Trashing the rim rarely wrecks the spokes - they're much more likely to go slack, which doesn't hurt the spokes. If you can get a new rim with the same or nearly-the-same ERD as the old one, you can just rebuild using the old spokes. That way you don't even need to re-lace the spokes - just tape the new rim to the old and move the spokes over one-by-one. I've done this a number of times and it's quite easy.
For calculating spoke length you need to know your rim's ERD, and also the flange dimensions of the hub, number of spokes/holes, and the cross pattern. Easiest way is to use Damon Rinard's Spocalc spreadsheet, which you can get here:
http://www.sheldonbrown.com/rinard/spocalc.htm