If you have changed the gpu and ram, you have already got your feet wet, why not jump in? I promise you building a computer is not difficult by any means. Just think of it as playing with legos. I mean honestly the hardest thing about building a computer is connecting the power button to the motherboard.
exept for having more ram and a better psu its a lot worse in performance. IMO building yourself really is by far the best option. You can also buy all the components and let the website put them together(I don't know where you live so I don't know which options you have but many websites do it for around 50usd)
yeah in Denmark you'll have trouble finding a website to do it or you. I assure you building yourself is very easy and takes an hour or two at most.
If you really don't want to though I suppose the first one build will do. 470 sli uses 700w on the dot under full load. Go with an OC or factor in psu degrading and you're in trouble. You could come by with it but I would get a higher wattage if possible, even if it means getting the 1kw cooler master. If this costs like 100euros extra though just get an xfx 850 and replace the cougar. Donate it to a friend or something lol.
I would choose an hd 6870 crossfire build over gtx 470 SLI, the hd 6xxx cards scale very well in crossfire, they are not power hungry and they dont run as hot, however I agree with Somebody 007 if you build your own system it will work out cheaper. You will usually find on prebuilt systems they skimp out on the PSUs and the motherboards and add lots of RAM as a person who doesnt know much about computers might choose the one with more RAM
also if you are not comfortable building your own PC you can always order the parts we suggest and when you get them take them to your local computer store and ask them to build it for you, I live in South Africa and they only charge R50 for that which is about $7