Upgrades to workstation to use as a server

I would like to modify a Dimension 4100 for use as a server (similarly
configured PowerEdge/Proliant exceeds my budget and doesn't allow for
quick/easy repairs). I would have a surplus of parts to draw upon if needed
from other Dimension 4100s. What are your opinions?

Max of 25 users, 25 clients, and 50K-100K hits each month on a website that
utilizes ASP scripting.

Keep the processor: PIII 1GHz
Upgrade the RAM: 2x256MB PC133 (512MB Total)
Server OS: Windows Server Standard 2003
Server Apps: IIS 6, Exchange 2003

The hard drive is a questionable area. I can use a simple 80GB IDE 7200RPM
drive (Western Digital 800BB if it makes a difference). This is easiest to
maintain/repair but might have issues in terms of performance.

I can make the jump to RAID-1 (boost in read performance and increase in
reliability) and/or I can make the jump to SCSI.

Does anyone know what controllers (RAID and/or SCSI) I can use with a
Dimension 4100 running Windows 2003? I would probably go with Seagate 18GB
15K or IBM 36GB 10K drives if I went SCSI.

From what I understand, the combined outputs on the power supply that comes
with the Dimension 4100 cannot exceed 140W. Should I expect to upgrade the
PS also if I go all the way with 2 SCSI hard drives in RAID-1 with other
devices that are typically found on a computer? Can a standard PS be used
in a dimension 4100?

THANKS!

--
Mike