Both ATI and Nvidia will be making AGP cards for at least another year and a half, maybe up to 3 years, depending on demand. Motherboard manufacturers are also going to continue making a home for them. As was pointed out though, the high end stuff is all going to be PCIe. AGP is going to be for the mid-range and lower stuff only.
As for 754, its experiencing a resurgence as the cheap desktop board, replacing the Socket A. The 939s are the high end boards, but with AMD producing a large variety of the new budget Sempron 64s in socket 754 format, it is the buisness-class board of choice for now. It will be around for awhile. Of course you give up dual-channel memory with the 754, and it is limited in how much RAM you can add. I think 2GB is the max on a 754 board, because they all have only 2 DIMM slots.