I've got a Ti4280(128mb - 8x) card, ok?
It is true that the games run slower if I set the card at 8x than 4x? why is
that?

That shouldn't be true. It should run "slightly" faster at 8X then 4X. It
doesn't seem to be always the case though. On my system my 8X card runs faster
then 4X with benchmarks. It probably just means my motherboard by design
focused more on 4X. 8X was just starting to be the norm when I bought my
motherboard.

Now about the AGP aperture size, I hear it's good set it at half of current
system memory. I have 768mb, so I put it at 512mb (rather than 256mb).
What's the best setting? and why?

No that's not true anymore. It might have been OK when video cards only had 16
or 32 megs. With todays video cards that have 128, 255 or I think even 512 now
there is not much need for system memory to be used. So with your 128 card, a
setting of 128 or even 64 would be fine. System memory is slower then video
memory so you really wouldn't want to use it unless absolutely necessary. Your
always better with having actual video memory.

That's why with games they will tell you not to use certain settings unless you
have a certain amount of video memory. They never even mention the aperture
size. Probably because it wouldn't help all that much no matter what size you
set it to because the video memory is faster and the aperture is very rarely
used..

So set it to 128 or 64 and forget about it. There is really no difference is
speed with any amount so I set mine to 64.