For this interested until we can safely release Rivatuner 2.25, here is the Overclocking and Monitoring Tool from AMD lets you overclock the HD 5870, also you can change the voltages and monitor sensors.
[0.9.25.1] July 17, 2009
- test build - disable device ID check on VBIOS images obtained via ATRM method
[0.9.25.2] July 21, 2009
- test build - Cypress/Juniper thermal sensors
[0.9.26] July 23, 2009
- release build with Cypress/Juniper support