Few simple fixes that save RAM, as static strings are stored in RAM by default.

Marlin/Marlin.h

 bool IsStopped();
 
 void enquecommand(const char *cmd); //put an ascii command at the end of the current buffer.
+void enquecommand_P(const char *cmd); //put an ascii command at the end of the current buffer, read from flash
 void prepare_arc_move(char isclockwise);
 void clamp_to_software_endstops(float target[3]);
 

Marlin/Marlin_main.cpp

   }
 }
 
+void enquecommand_P(const char *cmd)
+{
+  if(buflen < BUFSIZE)
+  {
+    //this is dangerous if a mixing of serial and this happsens
+    strcpy_P(&(cmdbuffer[bufindw][0]),cmd);
+    SERIAL_ECHO_START;
+    SERIAL_ECHOPGM("enqueing \"");
+    SERIAL_ECHO(cmdbuffer[bufindw]);
+    SERIAL_ECHOLNPGM("\"");
+    bufindw= (bufindw + 1)%BUFSIZE;
+    buflen += 1;
+  }
+}
+
 void setup_killpin()
 {
   #if( KILL_PIN>-1 )
     #ifdef SDSUPPORT
       if(card.saving)
       {
-	if(strstr(cmdbuffer[bufindr],"M29") == NULL)
+	if(strstr_P(cmdbuffer[bufindr], PSTR("M29")) == NULL)
 	{
 	  card.write_command(cmdbuffer[bufindr]);
 	  SERIAL_PROTOCOLLNPGM(MSG_OK);
       if(!comment_mode){
         comment_mode = false; //for new command
         fromsd[bufindw] = false;
-        if(strstr(cmdbuffer[bufindw], "N") != NULL)
+        if(strchr(cmdbuffer[bufindw], 'N') != NULL)
         {
           strchr_pointer = strchr(cmdbuffer[bufindw], 'N');
           gcode_N = (strtol(&cmdbuffer[bufindw][strchr_pointer - cmdbuffer[bufindw] + 1], NULL, 10));
-          if(gcode_N != gcode_LastN+1 && (strstr(cmdbuffer[bufindw], "M110") == NULL) ) {
+          if(gcode_N != gcode_LastN+1 && (strstr_P(cmdbuffer[bufindw], PSTR("M110")) == NULL) ) {
             SERIAL_ERROR_START;
             SERIAL_ERRORPGM(MSG_ERR_LINE_NO);
             SERIAL_ERRORLN(gcode_LastN);
             return;
           }
 
-          if(strstr(cmdbuffer[bufindw], "*") != NULL)
+          if(strchr(cmdbuffer[bufindw], '*') != NULL)
           {
             byte checksum = 0;
             byte count = 0;
         }
         else  // if we don't receive 'N' but still see '*'
         {
-          if((strstr(cmdbuffer[bufindw], "*") != NULL))
+          if((strchr(cmdbuffer[bufindw], '*') != NULL))
           {
             SERIAL_ERROR_START;
             SERIAL_ERRORPGM(MSG_ERR_NO_LINENUMBER_WITH_CHECKSUM);
             return;
           }
         }
-        if((strstr(cmdbuffer[bufindw], "G") != NULL)){
+        if((strchr(cmdbuffer[bufindw], 'G') != NULL)){
           strchr_pointer = strchr(cmdbuffer[bufindw], 'G');
           switch((int)((strtod(&cmdbuffer[bufindw][strchr_pointer - cmdbuffer[bufindw] + 1], NULL)))){
           case 0:
         int sec,min;
         min=t/60;
         sec=t%60;
-        sprintf(time,"%i min, %i sec",min,sec);
+        sprintf_P(time, PSTR("%i min, %i sec"),min,sec);
         SERIAL_ECHO_START;
         SERIAL_ECHOLN(time);
         LCD_MESSAGE(time);
   return (strtol(&cmdbuffer[bufindr][strchr_pointer - cmdbuffer[bufindr] + 1], NULL, 10)); 
 }
 
-bool code_seen(char code_string[]) //Return True if the string was found
-{ 
-  return (strstr(cmdbuffer[bufindr], code_string) != NULL); 
-}  
-
 bool code_seen(char code)
 {
   strchr_pointer = strchr(cmdbuffer[bufindr], code);
       int sec,min;
       min=t/60;
       sec=t%60;
-      sprintf(time,"%i min, %i sec",min,sec);
+      sprintf_P(time, PSTR("%i min, %i sec"), min, sec);
       SERIAL_ECHO_START;
       SERIAL_ECHOLN(time);
       LCD_MESSAGE(time);

Marlin/SdBaseFile.cpp

       // check size and only allow ASCII printable characters
       if (i > n || c < 0X21 || c > 0X7E)goto fail;
       // only upper case allowed in 8.3 names - convert lower to upper
-      name[i++] = c < 'a' || c > 'z' ?  c : c + ('A' - 'a');
+      name[i++] = (c < 'a' || c > 'z') ?  (c) : (c + ('A' - 'a'));
     }
   }
   *ptr = str;
 #endif  // ALLOW_DEPRECATED_FUNCTIONS
 
 
-#endif
+#endif

Marlin/cardreader.cpp

           SERIAL_PROTOCOLLNPGM(".");
           return;
         }
-        else
+        else
         {
-          //SERIAL_ECHOLN("dive ok");
+          //SERIAL_ECHOLN("dive ok");
         }
           
         curDir=&myDir; 
           SERIAL_PROTOCOLLNPGM(".");
           return;
         }
-        else
+        else
         {
-          //SERIAL_ECHOLN("dive ok");
+          //SERIAL_ECHOLN("dive ok");
         }
           
         curDir=&myDir; 
   }
   
   char autoname[30];
-  sprintf(autoname,"auto%i.g",lastnr);
+  sprintf_P(autoname, PSTR("auto%i.g"), lastnr);
   for(int8_t i=0;i<(int8_t)strlen(autoname);i++)
     autoname[i]=tolower(autoname[i]);
   dir_t p;
     {
       char cmd[30];
 
-      sprintf(cmd,"M23 %s",autoname);
+      sprintf_P(cmd, PSTR("M23 %s"), autoname);
       enquecommand(cmd);
-      enquecommand("M24");
+      enquecommand_P(PSTR("M24"));
       found=true;
     }
   }
  if(SD_FINISHED_STEPPERRELEASE)
  {
    //finishAndDisableSteppers();
-   enquecommand(SD_FINISHED_RELEASECOMMAND);
+   enquecommand_P(PSTR(SD_FINISHED_RELEASECOMMAND));
  }
  autotempShutdown();
 }

Marlin/stepper.cpp

   ENABLE_STEPPER_DRIVER_INTERRUPT();
 }
 
-int digitalPotWrite(int address, int value) // From Arduino DigitalPotControl example
+void digitalPotWrite(int address, int value) // From Arduino DigitalPotControl example
 {
   #if DIGIPOTSS_PIN > -1
     digitalWrite(DIGIPOTSS_PIN,LOW); // take the SS pin low to select the chip

Marlin/stepper.h

 
 void quickStop();
 
-int digitalPotWrite(int address, int value);
+void digitalPotWrite(int address, int value);
 void microstep_ms(uint8_t driver, int8_t ms1, int8_t ms2);
 void microstep_mode(uint8_t driver, uint8_t stepping);
 void digipot_init();