DCC
DCC (Digital) is becoming more widely available. With the introduction of the Bachmann Ez-Control, the Hornby Elite and Select systems, it is more affordable. (Other more established makers are available and their products are more reliable and offer considerably more facilities, but are more expensive).
The Improvements to the control of locos using DCC are not underestimated, Slow running, realistic Acceceleration/Braking is exceptional.
DCC also improves the pick-up capabilities of the loco itself.
Any loco that is running poorly because its mechanisms are old or faulty will not be improved by DCC Chipping.
Cost for a DCC System are the Controller (this controls the whole system), A transformer if not includede in Controller (or if higher total ampages is required - Running 5+ of locos at once) and Decode Modules (Chips) per loco.
All locomotives can be chipped (Fitting a Decode Module), the chip enables a loco fitted to be identified electronically and controlled electonically.
There are 2 criteria which must be meet before fitting chips:           
1)          The ampage to start loco (Stall Ampage Loading), and
          cruising when full loaded (Criuse Ampage Loading).
2)          The size of chip being used, against the space to fit it.   

Prices depend upon Stall/Cruise ampage that the chip can handle, the number of functions available and the physical chip size. Generally speaking prices go up with Higher Ampage, Higher Functions and smaller chips.
There are 3 types of Modules (chips) available + a sound Chip:   
 1)           Standard Chip which can have upto 16 functions, the  
                 chips are available in 2 function, 3 function and             
                4/8/16/32 function. Function 1&2 are forward/Reverse,
           3 for Lights and 4 for smoke. The others are for any
         other facilities excluding sound. (Costs £10 - £40)   
2)           As above but with the ability to support a Sound Chip. 
(Costs £35 - £80)                                              
 3)           As above (1), but with a sound chip built into the            
modules (Costs £75 - £150)                             
4)           Sound Chip (to be fitted to 2 above). This chip can be  
                programmed to hold any ound required. Many different
                 sounds can be held (i.e. whistles, steam ejector,               
               coalman shovelling, etc...), each of these sounds can be
 activated by the control unit. (Cost £55 - £100) 

General Costings:
'OO' Per Loco Standard from £10 - £40,
                   with sound from £65 - £100.
  'O' Per Loco Standard from £25 - £45,
                   with Sound from £75 - £150.

Fitting Charges:
DCC Ready:
We supply Loco & chip                                                                     Free
We supply Loco you supply chip                                                          £2
You supply Loco & Chip                                                                      £8
Non DCC Ready
Loco Testing                                                                                         £5
Chip fitting (X04's and Hornby Lima Ring fields                                     .
Later type (we supply chip)                                                     £12 + Chip
Chip fitting (X04's and Hornby Lima Ring fields                                      .
Later type (you supply chip)                                                    £15 + Chip
Chip fitting any other Loco. Price on application(POA)                  + Chip
H/Dublo/Wrenn Metal body locos High ampage chip needed          POA
H/Dublo/Wrenn Plastic body (Depends on pick-up/Motor fitted)     POA
All other gauges (excluding Z & N Not covered)                               POA
All above P&P at cost