You tried the manufacturer's site? They normally list the hardware specs somehow in detail. Some windows programs like Everest would let you find out detailed info about the hardware. You could also use a Linux live CD to get some PCI info too.
I cant recommend specific cards but USB/Firewire audio cards are usually better supported than PCI ones. Dont know about LAN, try to find one that says Leopard is supported, though it may be more expensive than a plain one...

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AsRock P45TS | C2D E8200 | GeForce 8600GTS
Mac OS X Snow Leopard 10.6.1 + Windows 7 Ultimate 6.1.7600 + Fedora 11