Best surround 7.1 card that's XP compatable

most soundcards will work with xp... as its still fully supported...
its compatibility with win 7 thats the problem as not all the manufacturers release new drivers for there hardware to force you to buy new...

creative has audioligy 2 and extreme gamer are some of the cheapest hardware rendering surround soundcards, asux xonar d1 also do ok surround and are relativley cheap...
dont bother with cards that generally cost less than $50 as they more often than not will use cpu to render the audio and on entry level machines you need all the cpu horses you can get...
so make sure you check for on chip hardware encoding...