|There are no such things as PDCs and BDCs in Windows 2008. They are all just DCs and (with certain exceptions) are all equivalent. If one fails then the rest just keep working.|
You can use member servers for file and print sharing, webs serving, database, etc. or you can also run those services on DCs, depending upon the load. I successfully ran a network supporting about 500 users with, initially, just two servers, both DCs, doing file and print serving. I wouldn't have thought that you needed to dedicate a whole machine to WSUS.
Personally, if I had, say 4 servers in all, I would set 3 of them up as DCs. Not only does this provide additional redundancy but it also shares the work of processing log-ons between the three.