Nope, in built graphics are only sufficient for low resolution gaming and playing 2-3 year old games. For the upcoming games like FEAR, HL2: Aftermath, AOE-III, etc, you need at least a 6200 128bit 128/256MB for gaming at 1024*768.
I remember having a Intel 845GV motherboard with on-board graphics and it could barely play a simple game like cricket 2004, let alone HL2. I have sold it a long time back. Even the performance for a strategy game like Rome:total war was not upto the mark.
So summary: go for a 128bit dedicated graphics card. The upcoming windows version, vista requires a DX9 card for all the visuals to display correctly. I am waiting for the day when people (read: my non-techie friends) who are satisfied with on-board solutions and gaming at 640*480 have to switch to windows vista.