Okay here we go ::takes a deep breath::
1) For better or for worse you aren't loosing that much performance by having your 9700Pro running 4xAGP instead of 8x; while there is a large increase between AGP, AGP2x and AGP4x.
AGP: Advanced Graphics Port
PCI: Peripheral Component Interconnect
And new to the past couple of years is PCI-Express (which is different than PCI-X).
2) The bandwidth between AGP, PCI, and PCI-Ex vary greatly; bandwidth being the amount and speed of information that can be communicated between a device using AGP/PCI/PCI-Ex and the RAM and CPU. (This is simplified)
In terms of bandwidth (high is better) the chain of command goes:
PCI < AGP < PCI-Ex & PCI-Ex x16
PCI, is used for a large variety of different computer inputs, for example you have PCI cards for:
-TV Tuning
-Sound Cards (Like a Sound Blaster)
-Video Cards (Like your 9700Pro)
-Port Replicating cards (cards the give you USB ports, FireWire, USB2.0, etc…)
PCI is very universal, however it is slowly being phased out in new computers (this also goes for AGP) by PCI-Express.
AGP, is used only for video cards. While some cards, like the ATi All-in-Winder series allow you to have TV tuning functional on this port, you cannot use this port for any other type of computer components.
PCI-Express (PCI-Ex), this is the newest component connection standard. There are two versions, a version used solely for Video-Cards and one for more general purpose (but can also do video-cards).
PCI-Express allows for much greater bandwidth than either PCI or AGP currently allows. This means that you can have cards with more function and/or that work faster than older generations. An example of this would be the nVidia 7800GTX series video cards, these cards are only available for PCI-Ex x16 and it is nearly twice as fast as some of the older top end video cards. (The 7800GTX crushes the 6800 series cards and ATi x800 series)
Most modern motherboard and computers (at the high end) will usually have a couple PCI slots, a couple PCI-Express slots, and then one to two PCI-Express 16x graphics slots.
To my knowledge there are no adapters to convert between these different ports. On the bright side, if you wanted to up grade your computer slowly, there will be some motherboards produced which have both PCI-Ex x16 (for graphics cards only) and an AGP 8x slot. So you could keep your 9700Pro until you choose to buy a faster card later.
(Sorry it's 2:02am EST right now, I just watched Tears of the Sun so I may need to edit this post later; but everything should be spot-on)
