$ E:/jdk14208/bin/java -verbose:gc -Xms32m -Xmx32m StringTest
[GC 2110K->96K(32576K), 0.0154693 secs]
[GC 2204K->98K(32576K), 0.0136285 secs]
[GC 2205K->99K(32576K), 0.0132961 secs]
[GC 2208K->101K(32576K), 0.0130327 secs]

$ E:/jdk5006/bin/java -verbose:gc -Xms32m -Xmx32m StringTest
[GC 2112K->144K(32576K), 0.0159576 secs]
[GC 2253K->146K(32576K), 0.0146236 secs]
[GC 2258K->148K(32576K), 0.0139504 secs]
[GC 2260K->150K(32576K), 0.0135361 secs]
[GC 2261K->151K(32576K), 0.0135612 secs]
[GC 2263K->145K(32576K), 0.0136109 secs]
[GC 2256K->146K(32576K), 0.0135520 secs]
[GC 2251K->154K(32576K), 0.0136070 secs]

in jdk 1.4.2, StringBuffer.toString()
StringBuffer.java
     585 public synchronized String toString() {
     586 return new String(value, 0, count);
     587 }

String.java
     562 // Package private constructor which shares value array for speed.
     563 String(int offset, int count, char value[]) {
     564 this.value = value;
     565 this.offset = offset;
     566 this.count = count;
     567 }

there is no char[] copy operation here.

But, in jdk 5.0, when StringBuffer.toString() is called,
StringBuffer.java
     585 public synchronized String toString() {
     586 return new String(value, 0, count);
     587 }

String.java
     200 public String(char value[], int offset, int count) {
     201 if (offset < 0) {
     202 throw new StringIndexOutOfBoundsException(offset);
     203 }
     204 if (count < 0) {
     205 throw new StringIndexOutOfBoundsException(count);
     206 }
     207 // Note: offset or count might be near -1>>>1.
     208 if (offset > value.length - count) {
     209 throw new StringIndexOutOfBoundsException(offset + count);
     210 }
     211 char[] v = new char[count];
     212 System.arraycopy(value, offset, v, 0, count);
     213 this.offset = 0;
     214 this.count = count;
     215 this.value = v;
     216 }

you can see copy char[] via System.arraycopy() that would incur memory increase.