With a .45, you can't do much better than modern equivalents of the old blackpowder loads--750-800 fps with a 255 grain bullet, if memory serves--but that'll do any job you send it to do, just as it did in the 19th century. It's and old design; anything heavier and you take a chance on blowing up the gun. I can't imagine anyone handloading for anything without a manual. Get one and follow the recommendations carefully--and be careful about using loads published in magazines. I once destroyed a perfectly good AMT Hardballer .45 using a load I got from a magazine.
If you're lucky enough to have a SAA in .44 Special, though, you're in luck. In that caliber the old Colt can take some very heavy loads (still, though, use a manual--they're in there). The late Elmer Keith pioneered what later became the .44 Magnum with heavy loads in .44 Colts and S & W Triple Locks.