Hey there.. from everything I've read and discussed both with Dell and outside Techie's (like me) and all saying the same thing.. No it's not easily upgradeable it's soldered to the board (granted how handy are you with a soldering iron) The Video Card is not easily accessible and is behind the main board (you have to gut the Dell to get to it).. once you find it you have to remove it with a soldering iron and replace it.. things to be VERY VERY concerned with.. One.. you lose your warrantee even if you have extended it you've lost it if you do this uplift. 2. It's really complex and not for the faint at heart, we are talking soldering chips first off and then onto the Motherboard. If you don't know what your doing here you can completely destroy your motherboard in a matter of seconds 3. Your uplift can only go as far as another GPU with the same power consumption / frequency requirements as the M640 GPU which depending on what you have in mind when you say "I'd like it to do so much more" I don't think you'll get a huge boost with any options available that are compatible. 4. Heat dissipation could be a major issue, it's already running hot with an M640 at full power it definitely can't handle any more heat being added to the inside of the chassis. 5. This machine was not built as a Gaming whore's toy, it's intent was for General Home / Business usage with some lighter gaming thrown in as a competitive edge over other AIO's.
So to answer the question directly - Yes you can (with a lot of work, loss of warrantee and possibly blowing your system from here to high heaven if you make one mistake during the uplift)
So the second question is - should you.. well :-) how deep is your pocket book? Do you care about the warrantee (this is one expensive toy to play with) - Personally.. NO YOU SHOULD NOT ATTEMPT THIS AT HOME :-)
The ball is in your court here.. high points:
1. Very complex
2. Need a soldering gun and lots of guts
3. Have to find another GPU that has similar power/frequency specs to work (not many options here)
4. Power supply in side the XPS from what I can tell is non upgradeable (I'll look into this deeper)
5. Heat dissipation is going to be a major issue (not much room inside the chassis to change the existing cooling mechanisms)
6. Loss of warrantee
Hope this helps.