I always check
www.pricewatch.com to find the best deal on a specific computer component.
More often than not, I end up buying from
www.newegg.com (I'm in no way affiliated with these guys). I built this PC about 9 months ago and got most of the parts from newegg. I spent about $1000, XP 2100+ (oc'd to 2.5ghz), 512 corsair pc3200, EPoX 8RDA+, GF4ti4200, antec file server, antec PSU, lite-on CDRW, 40gig WD 7200rpm, etc. (9 months ago, this was awesome, expensive stuff). They have great customer service, and ship fast as hell. I haven't heard anything about their European service, though, so you may need to look into that.
I would say unless you really need to do some serious number crunching, you stick with a single CPU setup. The dually won't be as good for gaming, if that is your primary interest.
Just my opinion.