Quite good, with a couple of reservations. Whilst they will work together, the CPU and motherboard are mismatched. The Asus Z77 mobos are excellent, but they are really intended to get the most out of unlocked Ivybridge CPU's. If you intend that, as an upgrade route, fine, although I doubt the benefit would justify the cost. If not, one, or other ought to be changed. Personally, if you don't want to use something like i5-3570K, and overclock, it is at least worthwhile getting one of the latest Ivybridge i3's. There would still be some doubt whether it was REALLY worth having a Z77 board, but it would, at least, a better balance.
The other point is, are you planning on adding a 2nd GPU, in SLi, in near future. If not PSU is way oversized. Stick with Corsair (or Seasonic), but 550W is plenty enough. Even 500W would be "adequate".
Case is a bit "qwerky", but seems OK, except lacking front USB3.
Cheers for that will reconsider the processor as reducing the PSU should give me a bit more cash to play around with, as for the SSD I could budget a corsair or M4 64G but 128G is more suitable as I know it's not wise to fill it over a certain amount. If I do go for a 64G how much should I keep free to maintain performance?
Generally 128GB is as much about value for money, as much else. Does depend a bit on your main hard drive. If it's good, and FAST, then there's no need to put more than OS, and software/games you use most, on SSD.
As to free space, I tend to work on keeping more than the size of my largest single file, although that may not be very "scientific".