No - bits is bits and there is no impact on the sound whether you use USB2, Firewire 400 or 800 or preferably SATA.
That said, given drive prices I would not bother with anything smaller then 200Gb and personally buy 300 and 400Gb drives. It turns out that the extra cost of casing and cabling a drive can be more then a bigger drive. In addition, fewer drives are easier to manage on many fronts.
I would specify 7,200rpm with a 8Mb cache though 2Mb will work fine. Make sure your case has a fan and a built in power supply. Use high quality cables - eg Belkin, Granite Digital, not for sonic reasons but for reliability. The better drives will advertise ballbearings and other noise dampening technologies.
You will find excellent drives by Seagate, Western Digital, Maxtor, Samsung, Hitachi. I personally favor the premium drives that are targeted at the "near line storage" or "enterprise" market as opposed to the consumer market.
www.newegg.com always has an excellent selection with some of the best pricing on the web.
Finally buy two drives - one for backup.