Your E7400 has the exact same core as the E5200 that I mentioned(wolfdale) so they OC about the same. Even with that stock cooler I'd bet you can get it up to 3.5ghz or close. With a decent aftermarket fan/heatsink you can probably get it up to 3.8-4.0ghz. The OC you can get is also dependent on the motherboard but if your is anything decent it should handle this fine.
I wouldn't recommend going beyond a better air cooler unless you have a trust fund and in that case throw that chip out and start building an i7 system
Here is the most popular cooler for the socket and it does a good job;
http://www.newegg.com/Product/Product.aspx?Item=N82E16835186134
There are programs to OC your processor from within windows but it's a much better idea to use your motherboards bios settings. The main setting you'll be messing with is the front side bus speed and also you will probably be bumping up the voltage of the cpu some for a decent OC. Going into all the details of OCing is beyond the scope of this post but there are tons of guides if you just use google or perhaps someone here can suggest a good one. Some of the guides can make it look more complicated than it is by going into insane detail. Try not to get discouraged if you look at some like that. It's really rather simple on most current motherboards and also safe with the safety features built-in to the chips now.
To OC the video card I'd recommend rivatuner.