By all means, AGP 8x is quickly being outmodded by PCI Express. I wouldn't be too sure on your question, but I would bet that they are. Some are speculating that PCI express, though not out long, will also be supplanted by new means of graphics interfacing
"Is a cheep pci express card better than a good agp card? "
No! An agp nvidia 6600gt for example will perform almost identically +/- 1% to a 6600gt pci-e card. Being pci-e does not make a card faster than an agp card. The reason cards are starting to go only pci-e is because super high end graphics cards built right now do not get the through put of what they can produce and therefore are limited by the agp interface itself.
If i were you, i would go ahead and buy a pci-e board, but buy a graphics card equivelent to the agp version you were thinking of buying. Middle of the road cards are always best for performance/budget.
I stand corrected, though little is mentioned of this in the stores when you're browsing. I would have suspecting, like using SATA vs PATA, you'd just have higher speeds inherent. It's always wise, though not alot of fun, to spend as much as you can on your graphics card... Typically cost equals performance in this catagory, but at the same time, they have a 6 month lifespan which doesn't bode well for your investment
It's not so much as a matter of speed (like how fast you are going down the highway) but how much bandwidth there is (how many lanes are in the highway). (More lanes of cars going constant speed equals more through put). Think of agp as a 4 lane highway and pci-e as an 8 lane highway. Both cars can go the same speed, but pci-e can get more cars through at that speed if it is capable (no cards utalize all of the available lanes of the pci-e highway yet). This isnt really an issue if the card is only using 4 lanes of traffic because the other 4 lanes would be empty anyways). It has only recently been seen in the nvidia's 7800 series that bandwidth is finally being exceeded on agp (ie there begin to become traffic jams in the highway). Thus it has less pipelines built into it vs its pci-e version soely because agp can't utalize them.