I wish there was a conversion rate for this. Unfortunetly, there is not. If you want my $.02, I'd say it like this : A Mac is an experience. Sure the clock cycles matter, but in what regard? Are you going to play games alot? If so, then maybe a higher end Mac would be needed. Are you going to use Word, or surf the net most of the time? Then ANY Mac will do. There really is no comparison when it comes down to it.
The ultimate thing about Macs, that I feel is better than PC's, is the experience, and the stability. My Mac rarely crashes, and my work PC bluescreens on a daily basis. My Mac is absolutely gorgeous to look at, and my many PC's are very bland, and ugly. My Mac performs the way I need it to, to get the stuff that I need to get done, done.
So in conclusion, to me it is more of a "Personal" conversion rate than a clock cycle converstion rate. Of course the faster the machine you can afford, with as much RAM as you can get in it won't hurt either.