Which hardware improvement will benefit the ISA performance the most?

I am configuring a new server for ISA and I need some advise to spend
the money into the right thing that will contribute to the overall
performance. Which hardware factor below is most important to an ISA
server?

1. CPU speed
2. amount of RAM
3. performance of the system drive
4. performance of the drive where cache file is writing to
5. performance of the drive where logs are writing to
6. Seprating the disks of logs and cache from the system drive
7. Other?

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