Yes, all AGP 3.0 8X cards are in compliance with the AGP 4X technology.
Did you read my post on the new nVidia 6200 w/ TurboCache? It is a great card for under $100, but sadly it is only PCI Express. Here is a solution to your troubles. Get the card below. It will not bottleneck on AGP4X too much, and is very fast, and very overclockable.
Only $75USD. That is about the price range you wanted to stay under I believe. Where do you live. I can easily have a Google Spider released to find all 5700 cards within that price range. It will only take about 10 mins. The spiders are fast (I have a friend who works on Google), and they find things unbelievably fast. [[[[<>]]]] Here Comes THE GOOGLE SPIDERS!! AHAHAH!