My dad recently upgraded from a 3 1/4 hp Porter Cable to an HSD spindle on his shopbot. The difference is night and day. Its a lot quieter and seems to have plenty of power. He only cuts wood, no aluminum but its definitely a step up. I would make sure to get one meant for fairly heavy cuts as the initial investment is $3-4k. You don't want to be replacing bearings in one of these, it will get pricey quick!

Matt