Since BestBuy links won't work after a bit I listed your system specs for others.
Pavilion Desktop with AMD Phenom II 820 Quad 8GB RAM NVIDIA GeForce 9100 graphic
Model: P6404y / BestBuy SKU: 9840522
BestBuy spec sheet shows Available Expansion Slots as 1 PCI, 1 PCI E x1, 1 PCI E x16, 1 mini cord (should be mini-card, not cord)
Which agrees with the HP website product specs
a single PCI-E x16 video card slot.
Crossfire is not an option for that model's motherboard.