Ok, just a point of view here. You want to buy i5 because it is cheaper than the i7, but you still want the best performance. Overclocking is possible, but if you start getting into overclocking levels which require advanced cooling (like watercooling), your cost savings disappear away to all the additional cooling hardware needed. So, keep your overall cost in mind, if that truely is a concern for you. You have to balance cost, performance, reliability, risk, and time involved.
You can use air cooling for low to moderate OC. Large heatsinks and large cpu fans. You got to make sure you have a proper case with adaquate cooling also. You also need to keep in mind the rest of your components and how much they can take beyond just overclocking a cpu also.
And you will be voiding many warranties, asking for potential trouble down the road (more heat can mean less reliability and less life), and could see some interesting data and system errors after extended use. I would never overclock a work or serious use computer that demands reliability. Only machines I would attempt that on are hobby, game, or systems I don't use on a daily basis and contain important information.
For information, this is a good site with forums and lots of tutorials and recommended hardware: http://www.extremeoverclocking.com/
This was a great answer on a previous question: http://superuser.com/questions/78223/water-cooling-a-pc-is-it-really-expensive/78246#78246
For great hardware reviews, performance tests, and information, it is hard to beat http://www.tomshardware.com/