1) Presonus Firepod
2) Edirol UA1000 + DMP3 or other stereo preamp
3) M-Audio Firewire 1814 + Behringer ADA8000 or other ADAT preamp
4) Hercules 16/12 + two stereo preamps of your choice.
5) Behringer BCA2000 + ADAT preamp of chioce
I've heard good things about the Presonus, I'm fond of Edirol's drivers, I think the M-audio product is good but I've heard things about it in terms of audio quality weaknesses, Hercules tends to be pretty good but I know very little about it, and Behringer is totally unknown quantity when it comes to driver support.