First, welcome from across the Pond!
Second, comments.
Case: definitely the CM690.
CPU: definitely the Q6600, for your applications.
Board: the Asus is a good board;
questions though: will you overclock? RAID setup? Firewire required? If so, a cheaper board could fit your bill, like the Gigabyte GA-EP35-DS3L.
PSU: good choice. Plenty of stable power for the Quad. The 550W could also do the trick.
Ram: good choice, though pricey for 4Gb. Then again, you're in Holland and prices may be different from here ($70 CAD:
http://www.ncix.com/products/index.p...Z%20Technology).
GPU: 4850 is fine, don't worry.
HDD: you could also go for the Seagate 7200.11 320Mb if you can find a better deal.
DVD burner: here in Canada, it's about $30 CAD.
Now, don't forget a good HSF for that Quad, to keep it cool.