so I own a Dell 6000 and am quite interested in similar threads. the 900 GMA frustrates me no end because despite was is written above. It's a pretty great GPU for an integrated solution. Now comes the kicker, Intel is really not so great at supporting the card and I'm still trying to find out why pixel shader 1.1 doesn't work on many games despite the fact that it is Pixel shader 2.0 compliant.
Anyway, on to the question at hand. I don't have Myth so can't test it, but I do have Max Payne 2. I've played it on the desk top, but loaded up on the laptop to see what it would be like. I bumped up the texture quality all the way and there's been no issues with lag yet.
As to the point on your card not having the AGP enabled, that is because the 900 GMA is PCI-Express and I doubt MP2 was able to check for PCI-E.
I've been contemplating starting up a site for the 900GMA on the Inspiron 6000 because of the many issues that could easily be avoided. It's a strong solution for an integrated shared resource GPU that needs to get some post design support to get the sort of respect it deserves.