You can install ESXi 4.1 on a laptop if you really want to. It won't be supportable, but it will work.
The important questions are;
1) Rackmount or standalone?
2) Supported by a Vendor or a custom build server to reduce costs?
3) How much do you care about your data?
4) Are backups important, what is your backup infrastructure?
5) Are you using local storage or SAN/NAS attached?
6) What is the clients expectatons around restoring services/data?
7) What is your budget?