I agree with the creative comment, stay away from them. HT|Omega is also good, look at the Claro II or Claro XT. If you are limited to PCIe, then you are stuck with Asus though.
Well it all depends on your budget.
But my suggestion would be anything from the Asus Xonar line
Xonar DX
Xonar Essence
Xonar Essence STX
From personal experience with several different model of creative cards,Stay away from Creative their drivers are really bad!
as far as i know any pci slot will work as long as it is 64 bit (which should cover anything since 945 chipset).What type of slots does the Claro XT fit? just any type of PCI slot or a specific type?