I Have some black triangle in the background instead of the stars and other moon or sun ...

I'm on an Imac Snow 500mhz, and the game still not lag :p

video card / driver funkiness.

Try changing the driver in-game. Failing that, make sure you have the most up-to-date driver for your card. Failing that, what card do you have?

Good ideas, but unfortunately not possible since he said he was on an iMac.
This is a known problem with older macs, usually ones with the Rage128 video card.

Try changing the Background detail in Options->Graphics. I would recommend a lower setting. Failing that, try lowering your texture quality in Options->Video.

Get a PC

(Hah! Couldn't resist!)

Ya sorry, I obviously didn't pay attention to the specs there.

Does he have ps_hack or the rage128 hack enabled? How about vertex arrays? Didn't the rage128 have a problem with compiled vertex arrays?

depends on the version of OS X. the ps_hack should be enabled by default if he has a rage128, but that's another thing to try.

Ok I'll try this