SSDs (well, the good ones anyway) are tremendously faster than hard disks at loading things, so by using one just for your OS and critical programs (which will fit on 40GB) your PC basically boots windows and loads these applications 3 - 5 times as quickly. A PC really does feel a lot faster with an SSD installed.
Ok, so these are the parts I managed to get. The MB was a slightly updated version and the graphics card got switched by a 2GB HD5870. Will it all work out nicely together or is there some sort of incompatibility, because of the different motherboard? Here are the parts already bought, (will get the PSU and case afterwards):
Should be fine. The 2GB HD5870 is a waste of money though unless you're using a large (30") screen (not an HDTV) or multiple monitors in eyefinity, as it literally offers no performance gains over the 1GB outside these areas.
As for the graphics setup (% compared to a 4870)
HD5850 = 155
HD5870 = 180
2GB HD5870 = 180
2xHD5850 = 155-310, depending on game (typically 280)
HD5970 = 160-320, depending on game (typically 290)