2. You can but I'd get 8GB just to be on the safe side. Anything above 8GB RAM is wasteful on a gaming build.
3. A Radeon 6850 / 6870 will be a far better choice in the price range than a GTX 550 will.
4. Yeah but I'd put a bit more into upgrading your motherboard. You sacrifice a lot of features by going B75 - go with an H77 instead.
5. I'd definitely look at swapping the motherboard for an H77, and I think you're paying too little for the GPU and the case. I'd put a bit more into both of those. The 550W PSU is fine as long as you're not looking to add a second GPU later on. That hard drive is also really slow and it will bottleneck your system - if you're using a mechanical HD go with a 7200 RPM one.
Yeah I don't think I will need any extras added in. Thanks a lot for the help though! I'm pretty much ready to buy just hoping it all fits together easily enough. Knowing my luck it will be a disaster but fingers crossed.