the pci-2.0 card work with pci-1.0 slots, but they run less power making the card run slightly slower.
pci-1.0 gives your gfx 75 watts of power, plus whatever you need to connect to it externally.
pci-2.0 gives your gfx 150 watts of power, plus whatever you need to connect to it externally.
my 4870 1gig pci-2.0 card is running half power right now, which means I'm not getting my full potential out of it. every card draws from the motherboard, just some need more juice, hence power supply's with pci express connectors, either 6, 6+2 or 8. each line gives 75 watts of power. so technically the 4870 runs about 300 watts, 150 from the motherboard and 150 from the psu pci-e connectors.
running half power will effect your new card purchase.
the 8800 is pci-1.0 so it will run full power with your pci-1.0 slot.