I recently over clocked my ATI radeon HD4870 and it caps at 75 degrees celcius at load and it idles at around 55, i was wondering if that was a safe temperature for my gpu
Yep, that is fine. My 4850 (which is the same GPU but at lower clocks) ran at 70-90C with the reference (design) cooler at stock speeds, so 75C OC'ed is quite acceptable.
If it wasn't safe your card would be producing visual artifacts. Some cards run hotter than others and idle temperature even for the same card varies between systems depending on the graphics card cooling solution and the case cooling and its internal components.