There are tons of companies that make great sound cards. I personally recommend Auzentech, although there are plenty of other you can find on newegg.com
Auzentech even has a sound card with an X-Fi chip, which means you get EAX and all the X-Fi features. ^^
You can resume games with other people, I haven't played since the last patch, so I'm not sure if AI can take over for missing players or not. It's usually not a problem though ^^. Just send the save file over to someone else and you can resume with them.