Woah!
Thanks a lot guys! Thats a lot of information in here and I'll surely consider it when doing some further research about the best buy.

Ok so it depends on the use of my system.
Well I'll mostly use it for gaming and game development, whereas I'm a scripter/programmer so I'll not use it for heavy rendering or high poly modeling.

The thoughts I grabbed from here are:
- Better get a 550 oder even 600 Watt power supply, as its long-lasting, and yes I currently have a 400 Watt enermax liberty and consider to buy either an enermax again or a "be quiet".
- Multi-cores are still not 100% supported, so a dual core with high speed should be sufficient.
I guess I'll take a look at the AMD CPUs, as I'm quite happy with my current CPU which is an AMD Athlon64 3000+
- I also consider taking a different motherboard, which might be somewhat more expensive, but longer lasting and better for upgrading in the future.

The problem with the motherboards is, that the P5K is one of the few motherboards having more than 2 SATA ports for drives. Most other motherboards have also more but thoses are connected to some sort of RAID chipset. So I'm not sure if they can also be used for single devices, as I'm not looking for a RAID solution.
My plan, or rather dream, is to grab a SSD (16 or 32 Gig, way enough) and use that one for the operating system (Windows XP Pro + maybe another OS in dual boot) and the system service programs, such as Firewall, Anti-Virus, drivers and all the stuff that is started on boot-up, to increase the boot speed.

The Data for programs, games and other stuff would remain on my two samsung spin points 160 GB hard drives.
Thus I need at least 3 SATA ports which are not RAID.

What do you guys think about SATA drives for DVD and DVD-burning?

Thanks a lot again!
Your input is really great!


edit:
I think an AMD Athlon64 X2 6000+ with 2x 3000 Mhz sounds good, as CPU cooler I consider a Zalman CNPS9500ALED

edit #2:
At a different place I could get the same CPU as box version for the same price as the tray version in another store.
Guess I'll rather go with the box version, as the fan included in that package should be sufficient, as I'm not going to overclock the CPU.

Motherboards: I have three options:
Asus M2N-SLI Deluxe
Asus Crosshair
Asus M2N32-SLI Deluxe
I guess I'll rather go with the Asus M2N32-SLI Deluxe which is more expensive than the M2N-SLI Deluxe but cheaper than the Crosshair and has the same features as the Crosshair plus two more USB ports.