Well, if you have money to spend, you might want to do so on 2 more gigs of RAM, only 2 gigs of RAM will severaly cripple a modern system. On topic though, the sound card technically sounds better than whats on the mobo, but this also depends on the mobo, if you can tell the difference, and if your speakers are good enough to not "bottleneck" the sound card. That psu isnt terribly good, and with your budget, the best you can do CURRENTLY is the 5770:
However, if you wait a little bit, Nvidia will be releasing the 450, 455, 440, and what ever they call them. The 450 seems to be a little cheaper than the 5770, approximatly 140-150, consume less at idle (my guess, since its downclocked so severly) and the performance seems to be on par or maybe +5%. So waiting for that might be the better choice.