OK, First my apologies for making Doc wait for me.....

First for Doc. The Black & Decker handheld vacuum which might come in handy in dusty situations....
[attachmentid=1640348]

The tools I used for this operation (Screw driver set, extra thermal pads, heatsink and gpu surface clearner and purifier, Artic Cooling Thermal paste, handwipes)
[attachmentid=1640352]

Firstly You'll need to have in pocession an ATI 5870 Card if you're switching to that. Also, you will need to remove all drivers from NVidia from your M15x using programs such as ccleaner or driversweeper. Once you've removed all and you restart, you screen will be like this but don't be alarmed as you're running what ever card which is inside but on default VGA display.
[attachmentid=1640355]

Turn the M15x over and place a sponge or cloth underneat the monitor as to not scratch it. Remove the battery and press the power button for 15seconds before opening up your unit to fully discharge it. You'll also need to REMEMBER TO DISCHARGE YOURSELF AND NOT TO WEAR CLOTHES THAT MIGHT BUILD UP CHARGE as a safety precaution. Read on Discharging yourself online just to be safe. I will not hold myself responsible for any error on those operating their units by themselves voiding their warranty etc. Do this at your OWN RISK!. Remove the screws circled in YELLOW and slide the back cover upwards revealing the whole internals of your m15x.
[attachmentid=1640359]

Once you've taken off the back cover, remove the 4 screws circled here to be able to remove the front panel in the next step.
[attachmentid=1640366]

You'll need to gently pry the front touch control panel using your fingers. Start from the right (star) and work your way to the left.
[attachmentid=1640369]

Now you've exposed the back gpu fan grill screws. Remove these 2 screws.
[attachmentid=1640371]

Once you've done that, turn over the unit again viewing the back portion. As you can see the fan is already removed here but u can remove yours by unscrewing the 2 screws in yellow. The red circle is the screw previously removed from the back panel/cover.
[attachmentid=1640373]

Here you can see the removed Fan grill and the fan as well. The screws circled in yellow are those to remove the GPU heatsink from the GPU. Remember to unscrew them slightly one by one before removing them fully altogether to ensure that removing the the heatsink will be easier. You will have to wriggle the heatsink slightly as the thermal paste and pads inside are sticking to it....
[attachmentid=1640374]
Now you will see your GPU exposed with the 2 screws circled in yellow left to be able to remove the card. Once you've removed the screws the card will pop up at an angle, gently slide it outwards: CAUTION!! DO NOT TOUCH THE COMPONENTS WITH YOU FINGERS AS STATIC CHARGES CAN SHORT CIRCUIT COMPONENTS.
[attachmentid=1640401]

Placing the heatsink and the GPU side by side, remember the position of the thermal pads as you'll be reusing them all alter. The areas circled in RED are those where the pads should be on which will be slightly similar in the ATI Card.
[attachmentid=1640415]

Here is the comparison of the NVidia GTX 260M(left) and the ATI 5870(Right). The arrows(yellow) are are suggestion as to where to transfer the square thermal pads from the NVidia card. The screws like structure in RED is GPU Mounting which are of different heights. You will need to remove the ATI Mouting from the back and replace it with the NVidia mounting from the 260M. Be EXtremely carefull and gentle. Don't have to rush thru. Use a screwdriver and slowly pry them out. This is a delicate process...
[attachmentid=1640443]

Height difference between the Nvidia(left) which is shorter compared to the ATI(Right).
[attachmentid=1640449]

Before applying the new thermal paste, I used ARTIC CLEAN 2 STEP remover to remove and purify the old paste and surface respectively. First step is to remove, 2nd is to purify. I did both steps at least twice.
[attachmentid=1640452]

I use ARTIC COOLING MX-2 Thermal paste. Try out the texture before applying....then you can use an old card to spread the thermal paste. Search online for videos for applying the paste on youtube. I applied a pea size drop and used my old driving license card to evenly spread it. I did the same for the GPU Chip and the GPU Heatsink.
[attachmentid=1640459]

A Closeup view of the chip. The areas circled show the reduced height of the GPU Mount after swapping with the NVidia card. Slot the card in the same way the NVidia card was taken out. Then screw the screws back in.
[attachmentid=1640469]

Here you can see both the heatsink and the GPU card before I finally replace the heatsink over it. The areas in RED are covered with additional thermal pads I purchased seperately as the ones from the NVidia card were insufficient to cover all. Once you've replaced the heatsink, do the identical steps above backwards till you end back at the beginning.
[attachmentid=1640471]

Once the new card is inside, boot up the system and the M15x will detect the card as a VGA Display. This is because the drivers are not installed yet. Download the ATI Catalyst driver from the AMD Website. ATI CATALYST 10.5 worked for me. Then install it and update the GPU Driver. Once updated the system will recognise the GPU as a 5800 series card and VOILA you have successfully installed the new card into your system. Remember to always practise caution and safety steps as to not kill the card or the system. Power should never be connected and the same goes for the battery. I used rubber gloves at some point during the process.....