Sins used to lag on my old rig:
3.2 ghz pentium D (dual core)
2gb DDR2 Ram
256mb 8600gts OC
And now it occasionally lags on my new rig:
3.33 ghz core 2 quad q6700
2gb DDR3 Ram
Two 512mb 9800gtx+ in SLI
The CPU profile while playing the game shows the core that sins is runing on maxed out, even though it is supposedly multithreaded.
If your system can handle it, my advice is to not look too hard at why... just sit back and enjoy...