Intel Q9450 should be faultless, and you've gotta tell me where you're picking one up for €280, as the best UK price I've found so far is £245ish (approx €324).
Motherboard choice is the only bit I'd choose differently, I have an Asus myself- about 5 years old- and it's been rock solid, but friends buying more recently have complained about quality and also compatibility. Couple this with their Technical "Support", which I know to be dire at best, and I'd point in the direction of one of the other brands- probably either MSI or Gigabyte. Personally, I'm likely opting for the Gigabyte GA-P35-DS4
(Link).
Antec should be good- got a P182 here, very happy.
Enermax PSU- also good, I went with a Corsair unit- 5 year warranty, and a modular cabling system, which means you only attach the cables you need- handy for when you need to improve case airflow.
As far as GPU goes, I'm a firm believer in not buying the latest/greatest unit- they always tumble in price, and there's always something "better" on its way next month. An 8800 GTS would have no problems at all with current games, a GTX or Ultra would be even faster, but if you have the money to burn then go for the best you can afford. Again, MSI would be my recommended brand for this- and their GTS comes factory-overclocked.
Good luck with the build.