I went from a ATI x300 to the 7800gtx, and let me say I have seen the light. From playing Fear at lowest settings to highest, playing BF2 on something other than low settings (I heard high might be asking too much). If you have the 6800 ultra, it probably isn't worth the upgrade, but if you have the 6800 go or the ATI x300, do it, do it now.
There are no safe limits to overclocking, if you overclock and it fries, your out the price of the card. For some people that's a drop int he bucket, but for a 360-420 dollar video card, personally is too much to gamble on.