From personal experience:
*At least a single 3GHz CPU, or at least dual 1GHz CPUs.
*1GB RAM works for me -- lo lag.
*A decent sound card, I recommend Creative Labs Audigy 2.
*I've edited on the cheapest of cheap video cards so I dont think this matters much, although I did splurge recently and get a Geforce 6800.
*Lots of disk space, I bought a 120GB recently and boy, was that a mistake. I filled it in a month. I've seen 300GB drives for $160. That's the best deal I've seen on the market. Get one. Or two. Or three. Believe me, you'll fill it up fast. (On this note, you'll want to get an external USB HD to backup on. If your PC crashes, you don't want to lose all of that data!)
*ONLY use Plextor DVDRW drives. By personal experience, they're the only truly reliable burners on the market. Don't buy Sony (like I did), the drives are so bad they can't even read what it just burned. (Dropouts, CRC errors... Sonys just make coasters) Spend the extra cash on Plextor. It's a good investment.
I hope this helps.