The Dimms slot is correct, u can double check cosz the two Dimms slot that 4 dual channel is same colour,usually.
If u put 2x256MB Ram n run in dual channel, is definitely faster than 1 stick of 512 Ram. But, remember to turn on this dual channel function~~some mobo need to manually on in BIOS settings.
R u not satisfy with u built in VGA?
I think is not a big problem is u want to change a ATI series graphic card. So far never any conflict about this.Well, u can try n let me know in case got any problems.