The "best bang for your buck" is the $189.95 you are going to spend on a Hotcams MudBuster camshaft. And then the couple of bucks for the center spring from a 660 raptor you're going to pick up from your dealer (or your favorite site sponsor/website/local guy, but not eBay, NEVER trust a part like this from eBay. I repeat, NOT EBAY! Under any circumstance. The guy on eBay could be giving them away with free shipping, DON'T DO IT! Don't even get the camshaft itself on eBay. Seriously. Did I mention NOT to get these parts on eBay?)

Here's the deal: You can add in aftermarket exhaust, it'll cost you at minimum twice as much as the camshaft (for a single) and more than 4 times as much for a dual setup. And you'll barely feel a difference in power with the exhaust.

The clutch kit will give you a bit of "seat of the pants" difference, but not more power. Just changes how the existing power is applied.
A "duner" EPI clutch kit is about $200.

Changing the sheave will give you more top speed, but not more power. If you like to go fast, this is not a bad option, it'll just take a while to reach top speed without more power. The Sheave is about $200 and up (but you send your old one back).

You could also opt for an aftermarket intake system, say AFE and it'll set you back almost $400 and you'll probably not notice a difference in power on a stock rhino. Same goes for just replacing your stock air filter with a K&N, it'll set you back $60+ and the seat of your pants won't notice a difference.

You could even play with changing the carburetor, coil, sparkplug, etc and you won't get the power/dollar that a camshaft will give you.

Mind you, a nice dual exhaust setup does give you a "bling" factor that a hidden camshaft will never give...