Depending on how many "customers" you'd want to have on it, you'd need to have a LOT of memory (to deal with all the website-requests - all of our webservers in work have 8GB of ECC (not cheap) RAM), quite a decent amount of storage space (if you're planning on having quite a few customers, you'll probably want to look into a disk-array - 16-shelf array, each with a 400GB SATA drive in it, gives you 6.25TB of storage space), and PLENTY of upstream-bandwidth on whatever connection you're planning on hosting it on (don't even consider hosting it on anything with less than 1Mbps of UPSTREAM bandwidth, if you're planning on having more than a few customers on it).

Once you've sorted the hardware, then you need to think about the software......are you going to stick with Windows, IIS, etc etc, or are you going to go for the more popular (for servers, anyway) Linux, with Apache/etc?