I have the Asrock with a e6300, DDR333 and x800pro. I would definitely recommend this board to anyone looking to upgrade while retaining ddr and/or agp vid cards. If you have PCI-Express the performance hit isn't as bad as I thought it would be. With my x800pro I am pulling 12486 in 3dmark03, with no complaints. My superpi is @29 seconds. Overclocking isn't it's strong point as the highest fsb you can expect would be around 310(stable). Overall I would say it's a steal for $50. By the way, my e6300 booted right out of the box even with the original shipped bios so don't worry about that. Don't forget to download the latest chipset drivers and bios and you're good to go. As for the x1900xt I'm not sure if it's compatible, a well placed email to Asrock would probably yield an answer. I would guess yes, but what do I know.
I've just been testing overclocking my 7900GTO, and when I hit about 725Mhz memory speed the 3D05 scores start to level off at about 10,500 which where (I am assuming the PCI-Ex4 Bottlenecking comes in), so a conclusion that can be drawn is that the PCIx4 will bottleneck a card somewhere between a 7900GTO and a 7900GTX, very roughly speaking.
So if you do happen to have a card that has a bit more power than this the losses you will get are going to be minimal.
To the OP: I think that a user called crone has a 1900xt (just checked my pm's and he did tell me he got one), so you'll probably be OK with it.
the list is incomplete, but people have foud other cards that work quite happily, its like any approved vendors list, they have tested what they have tested. I've emailed and asked for a non-compatible list which would be more useful, but they don't have one.