The Asus P7H55 M-LX has one pcie 2.0x16 slot so that will work for that video card. (pcie 3.0 is backward compatible)
Your issue may be that Saphire/ ATI requires a 500 Watt Power Supply and 1x75 Watt 6-pin PCI Express power connector. Also this card is just about 7" long so you'll need to check your case to see if it will fit. You didn't specify what power supply your currently using so you might need a new power supply too. Do not get a cheap 500 watt power supply for $30.00....They are very unreliable. Look for popular name brands and check out unbiased reviews before you purchase a power supply
Edited by Joe C, 26 August 2014 - 06:24 PM.