The 1GB 4870 will suffer from it's slower RAM. It's narrow 128-bit bus means it can use all the RAM bandwidth it can get. Thanks to the wider bus on the 3850 though it should pull out ahead most of the time. Just compare regular benches of the 3850 and 48701GB.
Well, with either you need to get the darn thing to work first
. Most of the drivers that run the AGP Radeon cards are not WHQL certified, which is a pain for 64-bit systems.