What size front tires should I use??

Those tires should be fine, as long as there are no huge rocks or ruts to bottom out on. Also for more turning radius, cut the stops back on the steering stem at the bottom. that will give you more turning radius, but don't cut them back too far because your tires will hit the a-arms and might cause you to flip.

You typically won't get as good of a ride with bias ply aftermarket tires - but a set of 22's will likely help.

You may find reducing the pressure in your holeshots restores some of your ride quality -- you can get away with less pressure in an aftermarket tire than you can stock, but be careful, if you run too little pressure you'll increase your likelihood of flats, it's a balancing act.

If you're coming off a 400EX, the 450R definitely won't have as good of a turning radius. About your only choice is going to be shaving some off the stops like the other poster mentioned, but just as he cautioned, make sure you don't take so much off as to allow the wheel to hit the control arm at full turn (check it at various conrol arm positions throughout the travel, if I remember correctly, full extension is where it's most likely to have a problem).

Personally I run 22" RAZR2's on my XC quad, if you want steering precision, they are hard to beat --- in many cases they almost have too much bite and can have a difficult to steer feeling in real tacky conditions.