You could always download a windows 8 preview, until you can buy an updated OS. Windows 8 will sell for 39.99 upgrade for XP/7 and apparently allow you to install windows 8 or windows 7. (according to Tom's article)
And I had the HD5770 (same card) worked great played Skyrim at high but only at 1600x1050, 1920x1200 was just too much in skyrim. BF3 Played great at 1600x1050 as well with that card just remember BF3 multi player is MUCH harder on your system than single player. I only played Single player.