Wondering about the same potential upgrade.
There are definitely people running 1st gen MP’s with 4870’s in them (both Apple branded and EFI flashed 3rd party PC cards).
I don’t have the link handy, but someone did do a few benchmarks comparing the 4870 and X1900 under Leopard and FCS2. They only saw a small speed increase at best.
I wonder if, as Michael suggests, the 4870 is running at PCI-e 1.0 speeds in the older MP’s and therefore doesn’t give you an advantage over the X1900?
I’d still love to hear about someone benchmarking the X1900 and 4870 in Apple Color on a SL, FCS3 MP1,1.