It really depends how you are going to use the server as to what you need. Occasional backups to a central location and some sharing of media files? 2GB of RAM, $65 Pentium G620 and a 320/500/640GB cheap SATA boot drive is fine. Do you want to also use it as a central media hub to multiple systems at a time, perhaps transcoding video on the fly? Then you will want to spend more on a low power quad core i5 and get 4GB of RAM and a solid motherboard. Think about how you will use it and search for what others are doing.
Typically for a home server you don't need what 15K SAS drives offer over SATA. They are designed to be continuously accessed 24/7, a home server won't do that and doesn't need the durability or performance. Enterprise grade SATA drives are a better choice than standard storage drives if you must have have something better, but again not necessary. You'd more likely use them in an office environment where multiple systems were accessing the server throughout the day. A cheap SSD with 40/64GB capacity would be even better.
Honestly there are a lot of web-servers running off pretty low level consumer stuff. Heck there are mac mini servers running business networks and websites.
You want to really be focusing your money on making sure you have great data redundancy or more simply put a good RAID card and lots of drives. You will need at least 3 drives for RAID 5 and total capacity is drives-1 x drive capacity, so three 2TB drives give you 4TB of storage.
There are plenty of issues both potential and past with RAID 5 and Windows Home Server. Just RAID 5 in general can be an *** so don't see it as some infallible place to put data. If it is just media you can get again perhaps it isn't so much of a consideration, but if it were for GBs of photos you might want to also back it up to another RAID 5 or just a collection of disks the same size. Or do it the other way around, a JBOD (just a bunch of disks pooled for large storage) array of 2TB drives backed up to a more reliable (in that two drives need to fail) RAID 5 array. 5x2TB drives could then give you 4TB of storage backed up. Needing 3 drives to fail to lose your data. Or maybe you just would want 8TB of storage for a media library.
Keep in mind that you aren't building a normal computer here, network connectivity and storage are the focus for performance. I'd suggest you have a look at what others are doing, you might even be able to find someone doing exactly what you are and see what issues they've had then copy their build. Also do a bit of research of Windows Home Server as it does have its own issues. Audio/visual forums and photography forums can be a great source for home server discussions as these people tend to need a lot of capacity.
http://www.avforums.com/forums/computers/
http://www.maximumpc.com/article/features/build_it_ultimate_windows_home_server