exchange 2000

The minimum requirements are:

Intel Pentium 200 Mhz or faster
At 2 GB HDD on Exchange drive
at least 500 MB on System drive
at least 128 MB RAM
Page file set to min of twice the system RAM
NTFS file system
12x CDROM

I would recommend:
Intel P4 1Ghz+
512 MB RAM
Your exchange drive should be (no of users * mailbox quota) + ~10%

You need to be a little more specific on your requirements. Are you using public folders, IM; Chat; or conferencing facilities? The more services the higher the spec required. The minimum spec is Pentium 300Mhz, 128MB RAM, drive space (no users * maximum quota)+ (space required for public folder, chat and conferencing services). You must have a minimum of 500mb free disk space on drive containing exchange, and minimum 200mb on system drive. These are however minimum specs, and in reality you will need at least the following: P4 Single processor ( a dual processor is preferable if you are going to exceed 300 users). 512MB ram. Use the equation above for hard drive size. If your environment is critical, and your budget permits, then use raid5 for your databases, and raid1 array for your transaction logs. This will provide good fault tolerance for the most critical files.

I agree on the hardware requirements in answer 1 & 2. On the amount of harddisk space i don't.

The volume on which you store the information stores should be:
(2,5 * (no. users * maximum quota + growth)) + (no. public folders * maximum quota +growth) + (space required for chat and conferencing services).
For information store maintenance/repair you need 1 to 1,5 times the size of your information store as free space. That's the reason the volume should be 2,5 times the size of your IS.
The growth is the expected growth during the 'economical' life of your hardware. If the expected growth is 5% /year and your hardware should run for the next 5 years, the growth factor will be 25%.

The harddisk configuration depents on your use and desired performance and fault tolerance.
For max performance and foult tolerance do the folowing.
System - RAID 1
Information Stores - RAID 5
Logs - Single disk or RAID 1.
For 150 users this is a bit over the top.
Just split information stores and logs, as below:
System + Logs - RAID 1
Information Stores - RAID 5

Good luck.