1. Is this at all a good choice?? (the price is not the matter)
2. What motherboard is a good choice for that??? (I want to oc a little)
3. What power supply can hold the system stable and works without problems?
4. What cpu cooler would be a good choice? Cool and silent if both possible.
Not really, it's just that it's DDR2 1066 and requires 2.1v to reach it's timings. It will be a slight bit faster once set in BIOS , but may cause a bit more hassle. Unless you're going for a serious overclock, the DDR2 800 would be fine for you. http://www.newegg.com/Product/Product.aspx?Item=N82E168...
1. I would change the RAM, Dominator is expensive and its 1066mhz which has no real benefit unless your a super clocking or want to run any other ratio than 1:1. As suggested by dirtmountain 800mhz will be great and cheaper.
2. P45 is a good choice still gives you crossfire if you need it as well Asus P5Q is a good range (P5Q-e is best feature/$$ board)
3. Corsair 620W is a nice choice agreed.
4. If you want silent you want a 120mm fan, and as suggested the XIGMATEK HDT-S1283 is one of the best out there and cheap others would be Thermalright Ultra 120 and Tuniq towers (you need a big case though.)