1 = I do it at 24bit/44.1. The extra bits mean you stand to keep more of what quality you get if you carry out any processing - noise reduction, levelling etc - on the digital recordings. When you're happy, you can convert to 16/44.1 for CD, but also keep the 24bit versions backed up as .wav files (or AIFF if on Mac) as masters. I don't think there's much advantage to using a higher sample-rate, but if you do, then 88.2 will downsample better to 44.1 than 96 would. Then again, 96Khz mastering does give you more options for transfer to future media formats.
Also, with 24bit, you don't have to worry so much about getting the hottest recording level. You can happily have the peaks averaging around -6db and so avoid any chance of clipping.
2= Given the noise levels and frequency response of tape, your current interface is more than adequate. If it's a professional archiving project, the budget and spec usually demands a top class external A/D converter. For most of us, it isn't worth that expense.
Above all else, make sure your tape playback deck has properly aligned heads (azimuth - a little spring loaded screw one side of the play head) and all heads, capstan and pinch roller are clean. I find I can set the azimuth by ear - it's the point where the audio sounds clearest, brightest and generally well balanced. When its badly wrong, the audio sounds dull and phasey. The azimuth screw often works loose in old decks, but leave it alone if it sounds ok and you aren't sure it could be better. Getting it badly wrong can twist the tape path and lead to chewed tapes!
