This is what you're after (click here_(live)) for the wireless adaptor and (click here) for the hard drive. I'm not saying you should buy from here, there may be cheaper deals around but it lets you know what to order. You will need a wireless router in order to connect the Xbox and your computer to the Internet at the same time though.
Just go to any computer game website and type Xbox 360 wireless adaptor into the search box. Same goes for Hard drive, there are 2 sizes as far as I'm aware 20GB and 120GB, I would go for the larger one as a 20GB drive can fill up pretty fast and they're not that much dearer.