If you want 8 inputs you can do it for under $400, but if you want 8 inputs with 8 mic pre's, then that might be a bit tougher. Your cheapest bet with decent gear might be an M-Audio Delta 1010lt with a Yamaha mg-series mixer with 8 inputs. If you buy used, you could probably swing that combo for around 400 (or maybe you could get it all new for around 400. . .)
You could also go with a card with 8 xlr inputs (mic pre's). The Aardvark Q10 has it, as well as something like
the presonus Firepod (which is firewire). There are some more out there, but I don't know them off the top of my head.
Another thing you could do that would give you way more versatility is go with a card like the E-Mu 1820 or 1820M. These cards give you 2 mic pre's plus 6 line inputs, so you could get that yamaha mixer and to into them, or you could buy several different outboard mic pre's, like an M-audio dmp3 plus some other stuff. The expandability comes with the ADAT lightpipe input on the card. You have your 8 ins from the card (the two mic pre's plus 6 line ins), plus you can get a Mic pre channel (like the behringer or the M-audio octane) and run that in via ADAT lightpipe, so suddenly you've got 16 ins. It's a bit pricier, though. The cards run about $400 to $500, and with the cards alone you've only got 2 mic pre's. As far as the A/D converters, the E-Mu 1820M has probably the best quality of all the one's I've mentioned. I'm running an M-audio delta 44 on a similar computer system as you, and I'd love to upgrade to
the E-Mu 1820M, or, even better,
the MOTU 828 mkII, but it's more like $750.
If you need anything else, send me a pm, or just keep asking questions. . . .There are lots of people way more experienced than me on this board, but maybe these suggestions will give you a place to start.