Seems like a pretty decent system to me, maybe upgrade the processor to a 3800, or OC it. If your looking for prices ive found www.newegg.com to have nice prices, and www.mysimon.com allows you to compare the prices from each site.
Well I don't really have enough money or the need for a dual core processor, and the 3500+ is a reasonable price. I can't really see the 3700+ making that much of a difference. I could be wrong, and by all means, tell me if that is the case.
It's not much of a difference, but it might give you that 'edge' feeling; knowing you bought the best you could.
Also, I recommend the x-530 speakers. They are only 50$, like 20$ more than the ones you're getting, but it's surround sound 140w peak.