More details:
It is a Dell Poweredge 1500SC running Windows 2000 server, and use dot net,
asp.net and iis 5 on a Pentium 1266 MHZ (dual capable I think- one processor
in now) and 1 GB RAM and Access 2002.  There is 460 MB free on the C:
partition and plenty on the D: drive.

I'd look at the code before i spent a ton of money on a new server.  Does the
page use data binding?  if so, rip that out.  its a huge perf hog.

also, think about restructuring your pages.  web interfaces are meant to
give high level data, or chunked data.  Too many times i've seen people write
a page that spits out 2000 rows of info.  try to implement paging instead.

also, try moving your access db off the server and test that out as well.  
that way your IIS process and access process arent competing for the same
single proc.

These are a few ideas.

MSAccess for 100-200 simultaneous users?
I'm not sure that MSAccess will scale to that high of a volume.  Generally,
10 concurrent users is the magic number with Access.  So, I'd probably not
go more than 100 simultaneous users.  At that point, you really need to move
to SQL Server.  I suspect that as you approch 150+ users, you are going to
run into some serious performance issues, if not database corruption issues.

As far as your hardware, determine where your bottlenecks are, and spend
money on that item(s).
Load up perfmon, and add % Processor Time, Average Disks Queue length (for
each drive that you use, don't use the "total" one).  Look at those
counters.  If CPU is generally really high or maxed out, consider replacing
the CPU.  If the disk queue is above 2.0 for more than a few seconds at a
time, consider adding more / faster drives.
I don't have an easy way to tell you to monitor memory useage, but you can
use Task Manager to monitor that reasonably well.

IMHO, all apps should be developed on MSDE, and then upsized to SQL Server
as the load demands.  That way, your upgrade is a simple backup and restore.
There's absolutly no code changing needed, only a connection string change
possibly.

Michael

Your existing server can more than handle the load.
Your el' numero uno problem is that you are using
Microsoft Access.  It will "not" scale to that many
concurrent users and act reliably.

You could easily upgrade to SQL Server Standard Edition
and run it on the same box if you don't want to buy a second one.

A couple of years ago, we had 30,000 users a day on a
server not as powerful as the one you have now.  The
server barely batted an eyelash...