Why do I get ORA-27102 ON WINDOWS 64 BIT if I increase the memory to 2.5GB BANNER
  
  
  
  Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit
  Production PL/SQL Release 11.2.0.2.0 - Production CORE    11.2.0.2.0
  Production TNS for 64-bit Windows: Version 11.2.0.2.0 - Production
  NLSRTL Version 11.2.0.2.0 - Production   Windows Server 2008 R2
  Enterprise
  6.00 GB RAM 64-bit operating system  


Current memory settings

*.memory_max_target=1536M
*.memory_target=1024M


When I try to increase the memory_max_target to 2.5 GB I get the below error on startup


  ORA-27102: out of memory OSD-00022: additional error information
  O/S-Error: (OS 1450) Insufficient system resources exist to complete
  the requested
  
  SQL> startup
  pfile='C:\oracle\app\product\11.2.0\dbhome_1\database\INITorcl.ORA';


Could some one shed some light.
How much real memory free does Windows task manager show, before you start up Oracle?  If less than 2.5 GB, that's the problem.  You can try increasing memory_max_target slowly from its current value, say in 256 MB increments, until it errors again, then I'd back it off by maybe 512 MB to give some headroom.