I'm not familiar with the 452 Silhouette model... but the Varmint weighs about 7 lbs. which means it will be tough to make Hunting Rifle class with a scope, rings and bases. The CZ rings are also fairly heavy. If I were buying my first rifle for smallbore silhouette, I'd get the American because I (and you) could shoot it in both Standard and Hunting Rifle class. I do this a lot when I shoot and it lets me shoot twice as much at a match... makes the trip worthwhile! You can get Weaver bases from Eric Brooks (
http://ebrooks77.50megs.com/) and then use Burriz Z-Rings (which are aluminum and lighter than the CZ rings) if you decide to go with the Varmint or Silhouette model.
Note that none of these rifles will necessarily be more accurate than the other from the bench. The advantage of the heavy rifle is that it holds steadier offhand. Therefore, most people who use standard rifles try to take advantage of the 10 lbs, 2 oz weight limit.
For scopes, I have had great success with the Leupold Vari-X III 6.5x20 EFR with the 1/8 MOA dot. It's spendy, but it works and works well. I have four of 'em mounted on different rimfires, and I won't buy any other scope.
Good luck!
