Is it possible to replace the GPU on a M6400...

..ok, so it is definately possible - I've seen the Dell guy do it to my machine when it went "phut".
I doubt I'll ever do it, cos I don't really play games and the Fx2700m gives enough power for what I do do - however I am curious - can you buy after market mobile GPUs - and if so, can you just slot them in like you would in a desktop?

 

unless im mistaken, the m6400 uses the 'mxm' graphics slot,

i do think you can possibly get after market mobile gpu's, although it is likely dell may have used a proprietary adaptation of the mxm interface and as such, you would be severely limited to those that dell produces and be lucky that someone may be selling a gpu better than yours on ebay or another classified selling service

 

I think what I'll do if I find a game that needs more than the FX2700m is buy one of those ExpressSlot=>PCIe adapter thingies that will let me plug in a real video card.

 

AFAIK, the M6400 does not use an MXM slot, but rather, a proprietary Dell slot. That means that your options are the 2700M and the 3700M.

 

It's a proprietary Dell slot, but you can get those cards from ebay.

 

The mxm ones?
There are some 3700M MXM for sale on ebay. Just don't know if they fit the dell heatpipe.

 

You can try laptop spare parts website.

 

I don't think you want to do that.

The expresscard's bandwidth severely handicaps mid-high end desktop gpus. If anything, you'll likely see a DROP in performance when going from 2700M to that.