Thanks everyone...especialy for advising me not to buy a already built up system but rather to assemble the system myself. I did follow this advice and have now a really slick machine...and it wasen't even that hard to find the fitting components and to build it up. I have now (for about 980$):
Mainboard: ASUS P5KR (FSB1333). This board also supports internal RAID settings
CPU: Intel E6750
CPU-Cooler: Arctic Freezer Pro7 (very effectiv and silent...and cheap!)
RAM: Corsair DDR2 800Mhz CL4 (2x1024Mb)
HD: 1x sATA Samsung Spin Point (320Gb), 2x IDE WD-Caviar (160Gb) my old ones!
GPU: ASUS 8800GTS (320Mb)
DVD-Writer: NEC(Sony) with DVD-RAM support
Case-Fans: 2x 120mm Scythe S-FLEX (10db...really super silent!)
I just played the Bioshock demo in highest settings (dx10 surfaces disabled cause of WindowsXP...) and it run absolutely smooth

The CPU temperature so far never got over 38'...system temperature (when playing Oblivion for about 3h) was at 41'.
Those components even seem to have a good OC potential...but I think I won't need that until Crysis is out

I can really recomend this System...it's powerfull, not too expensiv and super silent (if you decouple the HD's and activate ASUS's Q-Fans tec)
