The Card plugs into the PCI/PCI-e slot. Which is connected to what? You guessed it the southbridge or northbridge (depending on config). Admittedly it wont do as much work with the information but its not worth it.
would using a card for my sata HDD to plug into relive stress of a chipset rather usign the onboard sata controller?
it might. but it all depends on the card, and the board.
Usually all PCI 2.0 slots are connected to the South Bridge. And PCI-E is connected to the North Bridge. So having a PCI-E sata card will greatly imporve performance, because the CPU is also on the North Bridge. But no $30 card will help you. You will only see 25%+ improvement with Adaptec cards with 256mb of cache, which will cost you a cool $1000+