Civ IV was a break from "the way it was done" previously in the Civ Series, and those insisting playing it like previous versions declared it a dog. It is a more complex version and the graphics 3D rendering takes a lot out of your PC, but its the old story you dont get something for nothing. Purists will say 3D makes no difference, and in many ways thats true, but many lose sight of the purpose of the game - FUN, not endless bloodletting to be seen as the Worlds highest scorer. There is little doubt the 3D implimentation achieves what it set out to do, and the proper use of the 3D camera zoom and game table manouvering produces some eye catching effects.
I am a Civilization fan, although I wilted a little at the buggy initial V4 release. It is however stable now - has been for some time - and given todays entry level standard of PC will run fine, anything less and you will start to have a few speed issues. However I would always say dont hesitate, go get it - it makes a refreshing break between GalCiv games every now and then if nothing else.
It has far more depth to the game than GalCiv, but thats hardly surprising given the time the Civilisation series has been going. However for all that GalCiv is rapidly catching them over time, so Sid needs to watch his mettle - the rate these guys push out GalCiv updates is enough to make any Game Developer sweat

Which is better ? Hard to say, both come at TBS from different angles, and its like comparing chalk and cheese. Acknowledging the current lack of depth in GalCiv compared to Civ IV, my long term money is on GalCiv providing Brad keeps on going aka GalCiv III / GalCiv IV etc [ pretty please

] as I believe these guys have got the right focus and take enormous trouble to involve and listen to its gamers. Sid's guys are a little too distant for my liking, and since Sid sold it, they essentially now have to respond to Production House, Distribution and Revenue pressures. I think that the Civ series will wilt on the vine - in the end - allbeit a few years yet. Frogboy and GalCiv have the formulae right, and guaranteed longevity as long as their internal interest in it stays up - lets hope it does.
Regards
Zy