Sample configuration:

  <target name="mytarget">
      <java classname="my.packagename.MyMainClassName"
        fork="yes"
      >
          <!-- For remote debugging -->
          <sysproperty key="DEBUG" value="true" />
          <jvmarg value="-Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=8000" /> 
          <!-- For enabling JMX monitoring -->
          <sysproperty key="com.sun.management.jmxremote.port" value="8001" />
          <sysproperty key="com.sun.management.jmxremote.authenticate" value="false" />
          <sysproperty key="com.sun.management.jmxremote.ssl" value="false" />
          <!-- GC tuning. -->
          <jvmarg value="-verbose:gc" />
          <jvmarg value="-server" /> 
          <jvmarg value="-XX:+UseParallelGC" />
          <jvmarg value="-XX:GCTimeRatio=19" />
          <!-- Heap tuning. -->
          <jvmarg value="-Xms512m" />
          <jvmarg value="-Xmx512m" />
          <!-- JNI -->
          <env key="LD_LIBRARY_PATH" value="/my/so/directory" />
          <!-- Arguments, if any -->
          <arg value="my.argument"/>
      </java>
      <classpath>
          <pathelement path="${sample.dir}"/>
          <path refid="sample.classpath"/>
      </classpath>
  </target>

Nice resource for setting up Remote Debugging for Java.
Nice articles on tuning GC can be found here, here and here.
Java Ant Task here.