no the HD4870 only needs 1 PCI-E 16x slot, the card has a super big heat sink, so its thick enough that blocks the slot directly underneath it, be it a PCI-E slot or an ISA slot, or a nintendo cartridge slot... anyway, u'll want at least a 600w power supply to not have to worry about frying it all, even with a 4850.
here's a 4870
and a 4850
notice the 4850 is a lot thinner, so it only takes the space of one card, it being slow enough that it doesn't need the bulky heatsink. where as the thicker 4870 is so big cause of the extra cooling it needs, that it takes the space of two, even though they both only plug into one slot.