The 955 is a better CPU, but it's also a lot more expensive ($66). If you've got the extra budget, then go for it.
On the RAM: Both G.Skill and OCZ are great companies for RAM. In this case, the OCZ is cheaper than the G.Skill sticks with the same specs. If you were to step it up to 1333 mhz sticks, there's a good pair of Ripjaws with CAS Latency 7 for $100 that I recommend a lot. However, the OCZ sticks meet the specs and are cheaper, which is good for the budget.
I would check out the December GPUs by price. It's in the articles, under graphics. To be honest, I figured that you had about $125 to spend on it, and choose the one the article recommended. I would have liked to put an HD 5750 ($140) or 5770 ($155 for the cheapest) in it, but the budget wouldn't allow.
The case should have all the screws for mounting. The M4A79XTD EVO board comes with 4 SATA cables (2 straight cords, 2 90 degree bent cords) and one IDE cable. That should be enough for the general build. To be sure, just take a look at everything, and count what comes with it, and what you need. You should have everything there.
I found a combo with a slightly better Callisto CPU and the same board. I also squeezed in an HD 5750. You're a little over budget at $607, but I think this build is a lot better for only $6 more than the previous one.
I didn't spend a lot of time looking for combos, but there could be something else out there to lower the price even further. If you lower it by $22 more, you could squeeze in an HD 5770, which is an excellent gaming video card.