The current crop of PCI Express (PCIe) graphics cards are not too different from their AGP counterparts with the exception that they have been redesigned to communicate with the new PCIe x16 interface. The immediate gain achieved is the vast bandwidth that the PCIe x16 slot offers and of course faster transfers since PCIe operates at higher base frequencies than the AGP 8x slot. However, they aren't quite yet designed to take advantage of PCIe's advantages. The PCIe bus also allows concurrent bidirectional communication to and from the host controller with equal bandwidth. AGP too allows bidirectional communication, but that's at half duplex mode (unlike PCIe's full duplex mode) and its upstream bandwidth is insignificant compared to the AGP 8x downstream. For those of you who don't quite recall what full and half-duplex represent, the former allows data transfer in both directions simultaneously while the latter restricts data transfer at any particular point of time to a single direction. Here's a table to easily differentiate the key advantages:-
|Graphics Card Interface||Total Bandwidth||Bidirectional Transfers ?||Full Duplex Mode Transfers?||Max. Downstream Transfer Rate||Max. Upstream Transfer Rate|
|PCI Express x16||8.0GB/s||Yes||Yes||4.0GB/s||4.0GB/s|
In reality, current PCI Express graphics cards only perform limited upstream transfers because existing games and applications are designed based on the days of the PCI and AGP architecture. Back in June during Computex 2004, demos were being run to showcase the bidirectional bandwidth advantage of the PCI Express bus in the realms of high definition (HD) video capture, editing and playback of multiple HD video streams. Although this was something to look forward on the average desktop computer, it requires maturity of both hardware and software to realize this and at this point of time, it's not quite ready for mass usage yet. Besides, this would only benefit a select user group dabbling in high quality video.
Since the introduction of the first PCI Express graphics cards, both ATI and NVIDIA have been trying to put all that extra bandwidth into better use, such as going back to the early AGP days where its DIrect Memory Execute (DIME) feature allowed AGP Texturing. This allowed the use of the system memory to store large textures to be read by the AGP graphics card. Practical implementation of AGP Texturing in gaming however, didn't quite turn out as useful as was envisioned and graphics cards steadily bumped up local frame buffer memory to prevent the need to use that feature. However that's all in the past and with the new PCIe x16 interface, things are looking up as we venture into the first such useful implementation by NVIDIA that is now dubbed the TurboCache technology and it does a lot more than AGP's DIME.