First off, ‘ctrl+alt+u’ to bring up user preferences.

Under the ‘System’ settings make sure ‘VBOs’ is enabled.

Now start with the default cube.

In ‘Object Mode’ with cube selected hit ‘ctrl +1’ to apply one level of subdivision surface modifier.

'Alt + C ’ and select second option ( Mesh from Curve/Surf/…) to collapse the modifier.

Apply a Multiresoution’ modifier.

In the 'Mutiresolution modifier panel click the ‘subdivide’ button once while still in object mode.

!! Important : In ‘Object Data’ tab, make sure ‘Double Sided’ is off.

Switch to ‘Sculpt Mode’ and press ‘Subdivide’ in the Multiresolution modifier panel until noticeable lag occurs.

Stay in solid view

Do not use textures

Do not use the ‘matcap’ method

Do not use ‘texture solid’

At 6 million (9 subdivisions) I can still sculpt in realtime.

25 million (10 subdivisions) is useable -barely- with noticeable lag.

Anything higher and my entire system craps out.

The more ram you have the higher you’ll be able to go. I have 4 gigs. Video card memory and CPU don’t appear to be major factors.