Or does a cassandra cluster works best with the same
hardware in every node?
Things work best if the nodes all have the same HW. If you mix HW then you need to capacity plan against a machine made up of all of the lowest performing parts. 

What's happening with the poor performance ? 

Poke around with http://spyced.blogspot.com/2010/01/linux-performance-basics.html
See what's happening with nodetool tpstats ?
What sort of load are the nodes under ? How much data, how much throughout ? 
Have you changed any of the default setting ? 

I would guess with a single physical spindle you may have an IO bottleneck.

Hope that helps. 

-----------------
Aaron Morton
Freelance Developer
@aaronmorton

On 10/01/2012, at 3:42 AM, Daniel Bruegge wrote:

Hi,

I am currently running 4 cassandra nodes with the following hardware

- AMD Athlon 64 X2 6000+
- 8G RAM
- 750G hard disk

It shows not such a good writing performance and a really bad read
performance with sometimes also timeouts. I was wondering if it makes
sense to add 2 nodes with a different hardware (8 CPUs and more RAM)
to improve this. Or does a cassandra cluster works best with the same
hardware in every node?

Thanks & best regards

Daniel