zmeg - All the people I know (me included) have had no problems using the softmod. Get no artifacts or any other problems, seems to depend on make of 9800SE. Like I said the Powercolor Sapphires seem to be the best brand. Best to buy one from a store where you can return it, if you don't get artifacts, keep it, else return it.
SunKing - Yes they can by overclocking the cpu (no fsb on an Athlon 64 based system). But I have never seen the need for it with an Athlon 64 based system. Already get over 5.4Mb/s memory transfer speeds with DDR400.