Unfortunately, that is a PCI-Ex16 card (look at the additional images). Looks like a typo on Best Buy's site. I would also point you at
Newegg's listing for the same card which clearly states it's a PCI-Ex16 card.
Your power supply (if it's the one that came with the system) should be enough to power a card in the PCI-Ex16 slot (if I remember correctly, up to 75 watts via the PCI-E slot). It's only those graphic cards that require and extra power connector at the rear of the card that your system probably can't handle.
The 8400GS you linked would probably work without a hitch, but I would ask:
1) What's the purpose of the upgrade.
2) What's the budget.
3) If you look at the sticker on the power supply, it should state maximum power. What does it say?
-Wolf queries