Most onboard sound cards are decent for just gaming. I wouldn't worry about adding a card unless you plan on streaming movies somewhere and having the computer do the D/A conversions, not to mention on your budget. I ain't doin that shyt, I'm letting my Pioneer reciever take care of that.
For graphics cards, I'm a fan of Nvidia, and the brand EVGA. I think a GTX 550 should do pretty well, and I think they are like $100-150 right now. But again, I'm not super sure what will do that well on that low of a budget without building an entire computer. And, well, I just ain't got the time for that right now. Sorry :/
Just look for stuff that's on sale, sometimes the barebones kits on newegg and tigerdirect are pretty decent. You know everything that you need to get and how they are/need to be compatible?
---------- Post added at 10:32 PM ---------- Previous post was at 10:31 PM ----------
Oh....TOTALLY didn't catch that.
Pretty much forget everything I have said.