I assume that receiver is stereo/2.0 since you mention 1988ish.
External audio generally provides superior sound quality over internal PCI/PCIx stuff. I'm more into pro audio so not sure what a good entry-level external usb audio interface is. but i claim for 2.0, with good amplifier and speakers, a cheap external USB or firewire audio interface will sound better than comparable PCI/PCIx based stuff. It should def be possible to get something external at the price lower than the Asus Xonar DX.
I ended up going for the X-Fi titanium Fatal1ty. The shop didn't have Xonars in stock and reviews said it was about the same quality. Price didn't matter too much. Creative did piss me off with the Audigy2ZS/Vista drivers fiasco, but the X-fi drivers installed without a problem on Windows 7 so I forgave them.
Yeah my setup is 2.0. Just curious, why do you think a cheap external interface would be better? Less processing? (Also, I don't have any processing on the amp, it's just an amp with tone levels, no EQ etc)