Resolution:
The sun documentation states that Xmx to be > 2G will be to for Sun OS, but the max limit for the heap size varies among different operating systems such as Linux, WinNT etc.

Xmx max limits:
- 32bit Linux: ~1.8M
- 32bit Windows: ~1.7M
- Solaris (Spark CPU): quite unlimited (at least more than 3Gb)

** To make life easier for JVM, set Xms to be the same as Xmx
(there are some 'Hotspot JVM Performance tuning' on sun site which u might want to look at)