没有什么比回答你的问题的经验实验更好的了。
我编写了一个Java程序并在指定XMX标志的同时运行它(也使用XMS = XMX来强制JVM预先分配所有内存)。
为了进一步防止JVM优化，我主动分配X个10MB对象。
我在许多JVM上运行了一些测试，增加了XMX值，同时增加了分配的MB数量，在使用Sun和IBM JVM的不同32位操作系统上，这里是结果摘要：

操作系统：Windows XP SP2，JVM：Sun 1.6.0_02，最大堆大小：1470 MB
操作系统：Windows XP SP2，JVM：IBM 1.5，最大堆大小：1810 MB
操作系统：Windows Server 2003 SE，JVM：IBM 1.5，最大堆大小：1850 MB
操作系统：Linux 2.6，JVM：IBM 1.5，最大堆大小：2750 MB

这是详细的运行尝试以及分配类助手源代码：

WinXP SP2，SUN JVM：

C:>java -version
java version "1.6.0_02"
Java(TM) SE Runtime Environment (build 1.6.0_02-b06)
Java HotSpot(TM) Client VM (build 1.6.0_02-b06,mixed mode)

java -Xms1470m -Xmx1470m Class1 142
...
about to create object 141
object 141 created

C:>java -Xms1480m -Xmx1480m Class1 145
Error occurred during initialization of VM
Could not reserve enough space for object heap
Could not create the Java virtual machine.

WinXP SP2，IBM JVM

 
C:>c:\ibm\jdk\bin\java.exe -version
java version "1.5.0"
Java(TM) 2 Runtime Environment,Standard Edition (build pwi32devifx-20070323 (if
ix 117674: SR4 + 116644 + 114941 + 116110 + 114881))
IBM J9 VM (build 2.3,J2RE 1.5.0 IBM J9 2.3 Windows XP x86-32 j9vmwi3223ifx-2007
0323 (JIT enabled)
J9VM - 20070322_12058_lHdSMR
JIT  - 20070109_1805ifx3_r8
GC   - WASIFIX_2007)
JCL  - 20070131

c:\ibm\jdk\bin\java.exe -Xms1810m -Xmx1810m Class1 178
...
about to create object 177
object 177 created

C:>c:\ibm\jdk\bin\java.exe -Xms1820m -Xmx1820m Class1 179
JVMJ9VM015W Initialization error for library j9gc23(2): Failed to instantiate he
ap.  1820M requested
Could not create the Java virtual machine.

Win2003 SE，IBM JVM

C:>"C:\IBM\java" -Xms1850m -Xmx1850m Class1
sleeping for 5 seconds.
Done.

C:>"C:\IBM\java" -Xms1880m -Xmx1880m
 Class1
JVMJ9VM015W Initialization error for library j9gc23(2): Failed to instantiate he
ap.  1880M requested
Could not create the Java virtual machine.

Linux 2.6，IBM JVM

[root@myMachine ~]# /opt/ibm/java2-i386-50/bin/java -version
java version "1.5.0"
Java(TM) 2 Runtime Environment,Standard Edition (build pxi32dev-20060511 (SR2))
IBM J9 VM (build 2.3,J2RE 1.5.0 IBM J9 2.3 Linux x86-32 j9vmxi3223-20060504 (JIT enabled)
J9VM - 20060501_06428_lHdSMR
JIT  - 20060428_1800_r8
GC   - 20060501_AA)
JCL  - 20060511a

/opt/ibm/java2-i386-50/bin/java -Xms2750m -Xmx2750m Class1 270

[root@myMachine ~]# /opt/ibm/java2-i386-50/bin/java -Xms2800m -Xmx2800m Class1 270
JVMJ9VM015W Initialization error for library j9gc23(2): Failed to instantiate heap.  2800M requested
Could not create the Java virtual machine.

这是代码：

import java.util.StringTokenizer;


public class Class1 {

        public Class1() {}

        private class BigObject {
                byte _myArr[];
                public BigObject() {
                        _myArr = new byte[10000000];
                }
        }
    public static void main(String[] args) {
                (new Class1()).perform(Integer.parseInt(args[0]));
        }
        public void perform(int numOfObjects) {
                System.out.println("creating 10 MB arrays.");
                BigObject arr[]  = new BigObject[numOfObjects];
                for (int i=0;i <numOfObjects; i++) {
                        System.out.println("about to create object "+i);
                        arr[i] = new BigObject();
                        System.out.println("object "+i+" created");
                }
                System.out.println("sleeping for 5 seconds.");
                try {
                Thread.sleep(5000);
                }catch (Exception e) {e.printStackTrace();}
                System.out.println("Done.");
    }

}