We use several Mac OS 9 and Linux (RHL), I am just starting to investigate OS X, so I cannot reply regarding OS X server/client...
RE: dual duty:
I have a Mac dedicated to Mail server (gray G3/ 266Mhz) running EIMS (Eudora Internet Mail Server) under OS 9.2.2 nothing else runs on that machine. So far it has been plenty fast for email services (SMTP/POP/Spam and Virus Filtering)
I have several web servers, few 9.2.2 (one still on 8.6) running WebStar 4.5 (B&W G3 450Mhz SCSI RAID) and two Linux boxes (dual P3 1.4Ghz) running Apache, and also DNS.
I do not use the linux boxes for Mail, EIMS is more secure, much easier to configure/manage, and does everything I need. (EIMS should be available for OS X, if not now, shortly)
We also have a dedicated FileMaker Pro server, which acts as a backend to one of the WebStar servers.
Bottom line, the answer depends on your needs and server capabilities.
You do not mention which machine you are using, but I'd recommend that try it as is, with all services on one machine, and if the performance is not satisfactory, just add another machine and move some services around.
It should not be too difficult to move services from one machine to another, on linux/unix/bsd/osx most (if not all) config files are just text files that you can just copy from one machine to the other. (I've done it numerous times in the last 10 years...)
RE: Web hosting companies:
I would say yes. we host several of web sites, some getting over one million hits per day, and used to have a T1 line (1.54Mbps), however, it eneded up not being reliable enough, and not fast enough...
We had to upgrade to a dual fiber optics link (BBN and XO) with a total bandwidth equivalent to a T3 (45Mbps).
however, again, it all depends on your bandwidth needs. we host the web site of a Los Angeles news radio station, and we needed the extra bandwidth...
check the upgrade path/cost from the 512/256 to the faster connection, upgrade should be as easy as picking up the phone, just start with the slower/cheaper connection, and move up as needed.