Which is socket 468 ?I think u mean to say socket 478.Which supports the older p4.
The memory on a low end graphics card does not matter much.Mx 4000 has 128 MB of buffer while the 6600 also has a 128 MB of memory,this does not mean that MX 4000 that retails for around 1.5 K will be as good as 6600..
Tell us which graphics card are u takin about....