I have two systems I play on, both are XP systems.
Desktop:
Athlon XP 3000+ ~2.2 GHz
2 Gigs RAM
GeForce 7800GS 256 MB
I play at 1650x1050 with most settings on High/Highest. Frankly I haven't even tried them all on Highest yet. It would probably work great though, I just haven't felt unsatisfied with the way the settings are right now. The game doesn't skip a beat.
Laptop:
Core 2 Duo 2.0 GHz
2 Gigs RAM
Intel 965 Integrated Video
I play at 1280x1024 as that is the highest resolution that my external flat panel supports. The integrated video is setup to share 384 MB of RAM. The game runs really well even with the settings on High/Highest. When I zoom in close I can tell that the frame rate is not as smooth as my desktop and it has an occasional stutter when rotating the camera, but really, for integrated video it runs incredibly well.
Here are my conclusions on Sins requirements:
I would guess that having a strong CPU with a weak video card does make a difference so keep that in mind. The main thing is to ensure your system has enough RAM and video memory. Sins is not a CPU whore, and certainly does not require a powerful video card. I would say most PCs in this day an age can run Sins really well. Even if you have to crank the settings back a little it isn't going to matter so much. The game is played zoomed out most of the time where just a icons representing the ships and structures are displayed. Unless you are just zooming in to admire the beauty, you aren't going to stress anything on your system.
This is a great game that just about anybody’s system can handle. Do yourself a favor and pick it up even if you only meet the minimum requirements. When Ironclad suggested this would run on older systems, they weren't stretching the truth in any fashion. I would recommend upgrading to 1 Gig if you only have 512 MB of RAM. It is dirt cheap to make that upgrade and will benefit your system in everything you do. It will only set you back about 20-30 bones.