Alright after reading thoroughly again, graphics card are backward compatible. If it doesn't work, you may then require to update your motherboard. Read to find the details.
UPDATE: (After reading the comments I went back to researching again over this question and I must partly decline my statement of the PCIe configuration not being backward compatible. They in fact are very much compatible after a motherboard update (In some cases even the update wasn't needed. Sorry for the misguided information but I tried my best of reaching to the root of this controversy and reached a partially mistaken conclusion. So once again guys, if your graphic card doesn't work on your old motherboard then an UPDATE from from motherboard manufacturer will most likely solve your problem. My sincere apologies for any inconvenience or confusion caused. Thank you for all sharing your helpful experiences, as my final aim was to help ease the confusion and frustration that is involved with building a PC for the 1st time. Cheers to PC gaming!)