the 9800 pro rocks on that setup .. or you can go x800 / 6800 if ya can spare the $$$ .. ( 9600 pro id say is the best bottem line card for the $$)
as for the swap (ive swapped tons of cpu's )
turn it off .. flip off the power switch on the power supply , disconnect the monitor .
now i leave mine plugged in for grounding purposes / but that doesnt matter too much .. (its best to touch the case to discharge any static)
next pull the cpu fan plug .. then very carefully press the spring clip down and out w/ a screw driver ..(or down then take a second screw driver and pry it away just a touch and let up .. the heat sink will come off now .
take note the step in the heatsink base.
the metal/ or plastic bar now can be popped up and it releases the cpu .. it will be straight up from the board when its released . lift gently on the cpu and it will come off ( place in on a anti static plastic or insulated surface)

note : the arrow at the lower right .. this will match up to the corner of were the release bar pivots .. (on the asus a7n8x)
... really easy ..now just line up and drop the cpu in .. dont force it , but sometimes you have to push gently .. now while holding down put the lock back down (the cpu moves slightly when doing this)
Put a drop of artic silver on the center of the cpu chip ... and replace the heat sink ( affter All the old stuff has been cleaned from it ) remember the step goes over the white part that says "socket462" push lightly and twist just slighly and the compound will spread nice and even
now get the clip back on (dont slip here )
plug fan back in ... set ur stack back up .. ect ..
fire it up while holding the deleate key ..
go to "Advanced / adv chipset features
i think the stock setup for a 3000+
is 166 x 13.0 = 2.16 ghz .. but it should run in other combonations just fine. ( 200 x 11 ) or (200 x 11.5)
once this is set reboot and your are done except for checking the temp ..
use either the asus probe program or the hardware monitor in the bios ..
if ya did it right it should be below 46C
if its over 50c then somethings wrong .