I made a post a while back in the editing rig thread that basically answers this question (the PC specs part) here: viewtopic.php?p=1368015#p1368015
There have been some updates since then, so here's the new specs:
AMD Phenom II 1055T
Gigabyte MB with AM3, USB3, SATA3 (8 ports
), imbeded ATI Radeon HD 4290 (their info said 5870 onboard but my system reports 4290... huh...)
16GB of Corsair XMS3 DDR3 1333MHz RAM (4 4gig sticks)
Saphire AMD Radion 6800 Seriese GPU with Eyefiniti (currently powers 2 Acer 23" monitors on my desk and my 35" LCD TV behind my chair for video viewing, livestreams, and the occasional showing off my editing projects to myself and friends direct from Premier)
80GB Intel SSD (x2)
2TB Seagate HDD (x4)
Thermaltake Speedo Case w/ Thermaltake Frio CPU Cooler
Ultra X4 Modular 750Watt PSU
My sound card is Technically my Numark M1 USB DJ mixer board. It allows me to also capture my audio output. Probably the cleanest audio I've ever had as every PC I've had that I used onboard audio introduced ground hum along with vibration hum into the output line. Moving the card away from the system and going full digital (as the Numark uses a USB connection using ASIO drivers) ended that issue. Now I have tactile control over my volume, gain, bass, treble, ect...
Newer sound cards don't have this much of a problem and if you have a home theater receiver that you plug the HDMI from your PC into it'll also do the digital out to that through the video card (I use this on my home theater PC) and you can setup bitstream passthrough to allow you to send DTS HD and Dolby Digital HD right to the receiver and get full surround out from your PC (if you're playing a video source that has that.) But yeah, for most people I'd agree with Pwolf and recommend going with just the onboard audio. You really only need to go into special audio cards and what not if you are planning to go into pro audio type stuff.