You have a pretty good start - at $500 you have to be a real stickler with each dollar. So far, using the prices you listed (which doesn't include shipping) you're at $446 - that leaves $54 for an HDD and RAM which isn't realistic.
Here are a few things I noticed:
- Seasonic PSU are great and they're expensive. This Corsair Builders Series is ample for your build and is $40 cheaper and has a $20 rebate.
- The 6870 is a generation older - try the 7790. It's about the same price and comes with 3 games. If you don't want the games you could sell them to help offset the cost.
- The CPU is a good choice and the mobo looks pretty good too. If you need to cut costs, you can drop down to the FX 6300 which is $40 less.
- RAM is pretty standard so I would pick one that's cheap and has a lot of positive reviews on newegg. Oh, and you need 1333 (also called 10600). It's compatible with your mobo and doesn't require OCing.
- HDDs are tricky in that smaller sizes often don't save much money (if any). Be on the look out for a 1TB HDD on sale for $60 - it happens frequently and that's the going rate for a 500GB.
- Not including the CPU savings, you have $94 (54+40 from the PSU). 8GB of RAM plus an HDD is going to run at least $120 ($60 each). As I see it, you have a few options:
1. Drop to a lesser GPU (which I don't recommend)
2. Move to the cheaper CPU (a good option, but I think there's a better one)
3. Go with 4GB of RAM (I see it as the best option). You can pick up another 4GB for $30 down the road and is the easiest thing to install in a computer.
Hope this helps.