AGP to PCIe adapter

[ThinkPad560X]

How well does a AGP to PCIe adapter work? I got a few cards such as Nvidia GeForce 6800 256MB and many more around that era that are PCIe and if find a AGP version they are around $300+

I been messing around with some of my old desktops and I have a few APG Nvidia and ATi cards but want to use a little bit higher end of that era cards that went to PCIe only like on my Pentium 4 PCs that only have an AGP. Surprisingly some of my Pentium 4s have 2 SATA ports but still using the AGP slot.