I rendered 1.2 Milions polygons on Geforce 6600 GT with all effects (HDR, bumpmapping, soft shadows, etc.) in realtime. Now rendering just several milions in realtime (could be probably much more - I'm gonna try 10 milions and more) on Radeon HD 2900 XT with effects like radiosity, own PCMLSM shadows (physically correct), dynamic reflections and refractions, 128-bit HDR, depth of field, advanced physics using AGEIA PhysX, and much more effects (see Desert Improved demo on this site in Image of the Day topics - well this demo has less polygons, but i've got some with much more polys) using OpenGL and own realtime raytracer and AGEIA NovodeX - PhysX.

Vertex Buffer Objects, or just simply VBOs, are the last word in efficient storage of vertex data. They allow you to directly store your vertices on graphics card accessible memory (VRAM or AGP) so that the driver can push them through the pipeline exceptionally fast.

They're fairly easy to use and [google] is always a good place to start looking.
I know that DevMaster has a brief, no-nonsense, walk-through of using VBOs.
Or NeHe has a more in-depth, but as usual, convoluted example.
And when you want to get more intimate then try here.