As for the ATIs, the 9250 is my favourite, it's primary output is the VGA connection and bios screens output frequencies that a JPAC can manage. On the other hand the X1900 will be way faster but it won't do low dotclocks under Windows (no problem in Linux though). However, unless you plan to use some fancy emulators, the speed boost won't have any effect for MAME as everything is software rendered.
As far as I know, the 9250 was only available in AGP or PCI, which limits your motherboard choices somewhat. It's part of the R200 chip series and is a DirectX 8 part.
The X300 and X600 are slightly more advanced and were available in PCIe. They are part of the R300 series and are DirectX 9 parts.
I've seen X300 and X600 cards with the VGA connector as primary, but many (most?) have the DVI port as primary, so you'd have to use a DVI to VGA adapter dongle for those.
I don't know if the X300 and X600 cards output bios screen frequencies like the 9250.
Note that the X800 and later cards starting with X are based on a different chip series and are quite different than the X300 and X600. The only exception to this rule is the X1050, which is actually a juiced up version of the X300. They're pretty rare, and actually inferior to the X600 in my opinion.
I just picked up an X600 XT PCIe to play around with, and I think I have a 9250 PCI squirreled away somewhere. I hope to get some free time to experiment with them using CRT_EmuDriver and GroovyMAME in Windows to see if they behave differently.