1) not all the 6200's use shared memory. if you look at some of them thy do have 256 video ram memory. 300 core, 500 mem clock, 128bit, 4 pixel pipes.
2) some motherboard do run into compatablity problems if you mix differet kinds of memory sitck that have different timings, along with differen speeds... your probably just slowing your system down by installing the 256(pc2700) anyway. if you install anything lower then (pc3200), then your ram bus will run slower.
I guess its possible to run Doom on ultra since your card doesnt use shared memory. but thats probably on low resolution im guessing.
Im curiouse about how well doom runs on your graphics card. Do the following....
==1) in Doom3's Settings tell us what resolution its running at? example-(1280x1024?, 1024x786?, 800x?
==2) in Doom3 press 'ctrl + alt + ~' (~ is right under Escape). then in the 'console' type.... TimeDemo Demo1 1
...then hit enter, at the end of the Demo it shows you your average FPS.
==3) Post back here, about the demo resaults..... What settings+resolution it was running, and what FPS score it gave you.
(note) if you wont to enable FPS wile your playing, you can just type the following command into console..... com_showfps 1
i think, i cant remember exactly, if not could be... cl_showfps 1, Show_fps 1
typing any key such as 'C' into the console then pressing 'Tab' will show you the command list starting with that letter.