M-audio stuff is a good buy, no doubt about it.
Theres no disadvantage to getting USB or firewire unless you have LOADS of inputs running at once. Firewire only starts to give up if you daisy chain a few interfaces. PCI has no huge benefits for a low number of inputs (lke, less than 20). The best outboard soundcard (integrated interface, I should say: the DACs and and preamps in one thing) is p-r-o-b-a-b-l-y the RME fireface 800, and that, as the name implies, is firewire.
I think that EMU gear beats M-Audio at the same price, though. The DACs are better and the Pres are a bit sharper. I seem to recall they have lower, latency, SNR and crosstalk too.
Oh:
DACs; Analogue/digital-digital/analogue converters. A big deal. Top end standalone rack ones can set you back £2000, easily. Same with pres.
SNR: Signal to noise ratio
TLA: Three letter acronyms, and I hate em too.
Crosstalk: how much leakage there is from one stereo channel into the other.
Latency: self explanatory; the delay in processing a souind to hearing it and inputting it to it being processed. Less than 10ms should be expected from anything 'decent' (my 1616M gets 2.4ms), but that depends on the rest of your PC, too.