In order to upgrade to vSphere 4, an ESX host must have a /boot partition of at least:

A.
100 MB

B.
150 MB

C.
50 MB

D.
200 MB

Explanation:
vSphere Upgrade Guide ESX 4.0 ESXi 4.0 vCenter Server 4.0 vSphere Client 4.0 Page Direct, in-place upgrade from ESX 2.5.5 to ESX 4.0 is not supported, even if you upgrade to ESX 3.x as an intermediary step. The default ESX 2.5.5 installation creates a /boot partition that is too small to enable upgrades to ESX 4.0. As an exception, if you have a non-default ESX 2.5.5 installation on which at least 100MB of space is available on the /boot partition, you can upgrade ESX 2.5.5 to ESX 3.x and then to ESX 4.0.