Half Life 1 quality graphics are easily done in DarkBASIC Pro. Can't say that I have looked at Halo in great depth, but I should imagine that it is possible.
Remember though that the quality of the graphics largely depends on several factors which DarkBASIC has no control over:
1) Your modelling, texturing and general artistic abilities
2) Your programming skills
3) The speed and graphics card in the PC you are running the program on.
You won't be writing games with the visual quality of Half-Life or Halo in a couple of weeks. It takes time to learn. If you are interested, DarkBASIC Pro can load Half-Life & Quake 3 levels.