I had the same problem using a G4Ti4600. I found that if I toned down the AGP setting from 4x to 1x my system had stability. Now I don't mean tune down your AGP options within the XF86config-4 file, because I too did this and got nowhere. I lowered my AGP in my Bios and this seemed to help. I found that once I did this the system seemed to run longer before it hung.
I'm not sure if this was the correct cure, but I finally corrected my freezes by setting my bios back to its default settings. Since I've done this, Mandrake 9.1 runs stable and my OpenGl applications run like a champ. After doing this, I let my system run for 4 days, and it ran rock solid.
I have a Gigabyte GA -7VRXP mb and within the section that controls the graphics, I have options such as aperature size, strentgh of the signal and settings such as Ultra, Fast...you get the picture.
I had my aperature set as 128 and my setting at Ultra along with other things. Linux and Windows didn't seem to handle these setting very well and always resulted in a crash, infinite loop, or some other type of headache. Again, for you, I suggest setting your Bios to its default and then give Mandrake a try.
For your problem with the driver only seeing half of your video memory I don't have a clue. Sorry!
As for the agpgart, this should not be a problem for you. I might be mistaken, but I thought the problem only existed for users using AMD (athlon) processors.
More then anything it sounds as if your proplem lies within your motherboards bios. Best of Luck.