Yes, dells usually have pretty bad power supplies...
About the graphics card--I think your decision should rely on how cheap you can find a 9600GT. I am not certain of this, but I think the two of them together should be near a GTS 250/9800 GTX in general performance. They certainly wouldnt beat out any high performance cards, but would give you a little more power to run a lot of monitor space.
Found this in another posting:
http://www.neoseeker.com/Articles/Hardware/Reviews/palit9600gtsonic/
It looks like the SLI performance varies greatly from game to game.
Personally, were I you--I would rebuild my system first--spending more money on the PSU and processor. If at that point the added system power isnt enough to tide you over for a few months until buying a new graphics card you can consider another 9600gt as a temporary measure.
SLI does not give you double the power--maybe only 60% more, so if you can just save for a new card after finishing the build it would be more economical.
You might be interested in reading the system builder article from Sept.
http://www.tomshardware.com/reviews/amd-enthusiast-crossfire,2421.html
I dont know about euro availability on parts, etc. I hope someone else jumps in here to help you.
This forum looks similar to your issue:
http://www.tomshardware.com/forum/276145-31-budget-gaming-build
If you dont get a lot more responses here--design your ideal build and post it in the homebuilt systems section like the above person. You will get dozens of replies there with better ideas.