TechARP has published an article about Radeon HD 4870 overclocking. They used ATI Catalyst Control Center (or CCC) to overclock the card. The overclocking principle is to firstly to overclock the graphics memory with steps of 10Mhz until you see some visual artifacts. Once you see artifacts, downclock by around 10/20MHz. Second phase, overclock the GPU. Reset memory clock to default and increase GPU clock with steps of 10MHz and do the same artifact test than for memory.
Read the complete overclocking guide here: ATI Radeon HD 4870 Overclocking Guide