[T-Ray]

I bought a rem. 700 sps varmint last year (the dicks model) in .308 with a heavy barrel and the kinda weird stock that will be replaced later on. I now want to buy a "nice" scope for it to replace the el cheapo remington label scope that it came with. I really want to stay under $400, but i might flex if i need to spend more for what i want. The longest hunting shot i would ever take with it would be 300 yds, and the closest would be about 75, so i was thinking something like a 4x12 power, but i'm completely open to suggestions. I saw a cabelas scope that had the range adjustment knobs that could be switched out for different calibers, but i know that i'll eventually put something nicer on it, but i liked the concept. is there anything like this that is available in a better brand? I'm sorry that these are somewhat scattered comments, but this is really my first time buying a nice, long(er) range scope. Some questions: 40mm or 50mm? What power? Why do some scopes have what seems to be an extender ont the end of them? Would i have to get abnormally high rings for a 50mm? I was looking at a Leupold VX-II 4x12-40 for $450 on cabelas site, but want to know, would i have to get a ballistics chart and what not to be able to adjust for a long shot, or is there some "cheater" that it either includes or can be had? b/c i don't personally trust myself to actually aim above an animal that needs to be drt, not injured.