I have Xen installed with one virtual machine created, but I'm not
totally satisfied. The virtual machine was also 64-bit OpenSuSE 11.0,
just like the host.

Yast apparently will not allow me to get a screen resolution better
than 800 x 600 in the guest, and there is no sound. The Internet
connection works. The host hardware is fairly common: a K8V
motherboard with an AMD64 3200+ CPU, not virtualization-enhanced, and
a Geforce FX-5500 AGP card.

I suspect that Qemu would provide better support than that. (I once
had it running at 1280 x 1024 with the same display adapter.) Anyhow,
how can I tune Xen for better performance?