Mentioned the M-Audio 192 soundcard. An excellent choice but over here, virtually unobtainable now and very expensive. The Delta 2496 is almost as good, it will certainly be fine in terms of latency since it has only been in the last few years that external AIs could match PCI cards for speed be they usb or Fussywire.
AIs with simple analog input monitoring can exceed any PCI card in terms of low latency. An analog mixer gives a PCI card interface the same advantage, so since you have to have a mixer with many PCI cards why not use its inherent zero latency ability? It seem odd to me to have an obvious, easy and better solution in the mixer and still use the DSP input monitoring.
Yes! Of course you can use a mixer to give zero latency monitoring (we did not, after all listen to the off tape signal from us Revoxes!).
But if you need to use onboard FX or play a VSTi you need low, round trip latency.
BTW. The PCI card IS beat! My 2496's only go down to 64 samples (but are left at 256). My NI KA6 can equal that and the RME AIs can do 32 without glitching.
I'm in the fortunate position of never using live VSTi instruments (Audition can't even handle them) and think trying to use live effects while recording is a very bad idea even before latency is considered. Everyone's needs are different but I can't help thinking that many people make a rod for their own backs by overloading their systems then complaining about latency.
There are certain very good reasons to have a mixer but the suitable mixer is rarely a small economy one. My biggest reason for using a mixer is more to do with routing and monitoring options, not specifically tracking or mixing. Cheap mixers are almost always limiting in this area. Also, with the utmost respect to ecc83, I think it's wrong to say that the pre amps in small mixers are superior to most interfaces is just plain wrong. Yes, there are good--and more expensive--small mixers but I've yet to hear a pre amp I'd give studio space to on the entry level stuff. It only stands to reason. If you spend $150 on a box that's just a couple of pre amps and AD/DA converters, the manufacturer can put in more quality that a $79 box trying to be an 8 channel mixer plus interface.
So...at the risk of sounding condescending, if you haven't thought through why you want/need a mixer and researched whether the model you're considering can do what you need, they you likely don't need a mixer.