Which resampler should I use? Foobar has the PPHS but I don't know if it's the best.

I have the audigy 2zs, that seems to use 24bit@48/96khz. I'm not sure though.

Also, how should I set the playback-input-output to? For 16bit and lower there is the "dither" thing, and for 24bit and above there isn't.

Finally, what is the "ultra" mode for the PPHS?

Definitely use 24-bit output if you have it. Dither is theoretically preferable than no-dither, but 24-bit undithered is better than 16-bit dithered. (Both are very good).

For the audigy, Resampler is necessary for decent quality and PPHS is very good for resampling 44.1 kHz to 48 kHz. You can ABX its output and should find it's indistinguishable from a more processor-intensive resampler like SSRC because it's designed to be transparent to audible limits or better.

Ultra mode may be useful to go beyond audible limits of accuracy but is processor-hungry. You might turn it on when you're converting to a file or CD that might conceivably be put through some extreme DSP or EQ that might reveal differences you wouldn't get during live playback, but otherwise it's a pointless performance drain that might make stuttering audio more likely.