Line-in recording better through Audigy or X-Fi? XP or Vista? 44.1KHz or 48KHz?

All recent Soundblasters from the Live & up were made with 48khz in mind, 44.1k is internally resampled. X-Fi has six billion teraflop quad core chips on board because it needs all that power for proper resampling, especially since Audigy was sometimes criticized for its horrible internal resampler (not sure about Audigy 2 and later).

For HQ recordings, its a good practice to use as high of a sampling rate and resolution as you can, and convert back to 44.1k AFTER you edited your tracks. Preferably using some quality resampler tool (PPHS in Ultra mode perhaps, look up on Hydrogenaudio).
It's like when you color your drawings, you do it in ultra high resolutions to have less artifacts & a smoother image once finished. Doing all sound effects in higher resolutions (24-bit 96khz) can yield more accurate processing, mathematically.

And if you want high quality recording, I'd advise using a card like the Terratec Aueron or Audiotracks Prodigy; they are pretty much audiophile quality cards, both in playback as well as in recording (whereas the Soundblasters only focus on EAX, games and everything marketable, quality being an afterthought)

Of course if you dont give a fuck, just use 44.1k.

Jam it back in, in the dark.