Oh yeah, that was my last foray into overclocking my video card. A few frames per second isn't worth having to replace my board and a comparable board will cost me $300. I got mine in Oct 2009 for the unbelievable price of $125 and it has 1 gb of DDR5.
Don't let that incident stop you from gaining some extra performance from your 4870, just be sure enable manual fan speed control in the Catalyst Control Center or in RivaTuner and then up the fan speed to at least 85% before starting a gaming session; and then manually reduce the fan speed when done gaming.
The 48xx series of gpu's do not like to be overheated but the stock fan is more than enough to keep it cool (even if oc'd) providing the you manually up the fan speed to dissipate the heat. Do not let the driver determine/control the fan speed because it never ups the gpu fan enough to compensate for the heat generated from oc'ing or extended gaming sessions.