dont forget, the 9800 gx2 uses one pci-e slot with 2 high-perf gfx cards, if you have pci-e 2.0 it may not be much of a problem
then theres always microstuttering since sli technology has its downfalls, also some games may not good coding for multi-gfx card usage, same as some games run better on dual core rather then a quad core
the gtx 280 shines in high detail at high resolutions, beating a 9800gx2, now think of a gtx 280 sli, or even tri-sli, but then again, you only want to go to 2000 euro
I would just get a 74gb raptor and a 500gb or 750gb storage drive, but thats only since i deal with a lot of media files, im already at the 2 Tb mark
the raptor is a good drive as i own one myself, has a good response time and files load quick
since its your first build, its may be too much to go into watercooling, but you could be a fast learner; im still on air cooling since it works enough already, id only switch to water if i wanted to overclock higher, but again its ur first computer so you prob wont be doing that right at the start
if u have the extra money, just get a 9450, its 45nm (6600 is 65nm) better temps, but then again you're on water, and the 9450 has a larger cache, 12mb
just what i think an ideal comp would have...