The Final PC system requirements has been posted on 2K Games forums. It looks like 2K Games did a good job in optimizing the game so it won't ask for too much.
Windows XP (with Service Pack 2) or
Minimum System Requirements:
CPU: Pentium 4 2.4GHz Single Core processor
System RAM: 1GB
Video Card: Direct X 9.0c compliant video card with 128MB RAM (NVIDIA 6600 or better/ATI X1300 or better, excluding ATI X1550).
Sound Card: 100% direct X 9.0c compatible sound card
Hard disk space: 8GB free space
Recommended System Requirements:
CPU: Intel Core 2 Duo processor
System RAM: 2GB
DX9: Direct X 9.0c compliant video card with 512MB RAM (NVIDIA GeForce 7900 GT or better)
DX10: NVIDIA GeForce 8600 or better
Sound Card: Sound Blaster® X-Fi™ series (Optimized for use with Creative Labs EAX ADVANCED HD 4.0 or EAX ADVANCED HD 5.0 compatible sound cards)
Important Note: Game requires Internet connection for activation
That "Important Note" is something that bothers most of the users, but piracy is a big problem for game publishers. Those without an internet connection will probably get an "activate by phone" option.
The game looks great, we just need to wait a bit longer, since Bioshock is scheduled for the 21st of August.