Technically, you can use 6 GB on that motherboard, but you generally would not want to. The i5-750 operates in dual-channel mode, meaning that you want either 2 or 4 sticks of RAM in the motherboard, not 1 or 3 -- that would force you into single-channel mode. You'll get somewhat better performance in dual-channel.
6 GB kits are typically for 1366 motherboards, and processors such as the i7-930.
I think your build above looks decent, though I believe that this
Corsair XMS3 4 GB kit for 120 euros is the best price/performance available on that website.
I'm unfamiliar with the power supply you selected. Based on the
info I've been able to find, it appears to be a good brand, but you could get an
Antec Earthwatts 650 for less than half of the cost if you're trying to save money.
The only other thing I would change is the motherboard. The
ASUS P7P55D-E Pro is widely regarded as the best socket 1156 motherboard to get, as it allows Crossfire at 8x/8x (the best available for 1156), it has USB 3.0 and SATA 6 Gb/s.
EDIT: My head hurts a little from trying to read & search in Dutch.