First of all, modding of an XBox is illegal and will void your warrenty.
If you still want to mod your Xbox, you first need to figure out what revision of XBox you have because different versions of XBox requires different versions of mod chips. You can check it out here.
After you know your XBox version, I would recommend getting the Xecuter 3 mod chip from team-xecuter.com which is considered the best mod chip around. It also has a solderless adapter so you don't even need to mess around too much with soldering, just open your XBox, follow the instructions and you are good to go (the instructions are on team-xecuter site. Some even comes with complete pictures of how to identify and put things).
In team-xecuter.com you'll find a list of reseller sites, so I suggest you browse around there for the prices, but generally its about $50-$80 depending on the chip and if you are taking the solderless kit.
If you buy a solderless adapter which does not come as part of a kit, be sure that it is the right version for your XBox.
