Welcome.

You should get at least 370 KH/s (MH/s) with overclocked 5870.
- Use MSI Afterburner to overclock it.
- Watch your temps (max should be 90°C).
- Lower your mem clock to 300mhz (if you mine bitcoin - not a good idea at this point) and OC your GPU.
- If you get too many stales, lower the clock or increase your voltage on GPU.

If you are trying to mine scrypt coins - OC your Vmem with your GPU.