There is no reason why this card wouldn't work in a Dimension 3000.
That machine has 3 PCI slots, so just put it into the first one (the
one furthest from the bottom of the motherboard. There is a setting in
the BIOS (f2 on startup, i think) for Primary Video Controller in the
Integrated Devices menu. If that is set to Auto, then on startup the
system should look for a PCI video card, then an AGP video card, then
the onboard video card - so your video card should work straight away.
If it doesn't there's a good chance that the system has been designed
to ignore that setting in the bios (which is a default setting for
most of the Dimension range) because there's no AGP slot in the system
and PCI video cards are not in common use. If that's the case there's
little that you can do. You might be able to find a customised
user-made bios on the internet but that would invalidate your
I'm not sure which bus the onboard intel video card is on, but if it's
the AGP bus then there's a possibility that you won't get much of a
performance boost out of the PCI ATI card. Download a program like
3dMark 2003 (2005 is too demanding) from www.futuremark.com and run it
on the system with and without the ATI card, if the score is higher
for the system without the ATI card then it's not worth your while
having it in the system.