Hi,
I am running KDE4.3 on openSUSE11.1 x86_64.
It is consuming 1758 MB of 1914MB (2GB RAM).
How can I figure out and fix the problem??
How did you arrive at that figure?
Unless you got some software that has a memory leak then you should be fine, the rest should be just cache 
using free command:
mukul@linux-w5lp:~> free
total used free shared buffers cached
Mem: 1800692 1768396 32296 0 174452 653028
-/+ buffers/cache: 940916 859776
Swap: 2104472 16 2104456
mukul@linux-w5lp:~>
How to find this??
The free output seems normal.
See mine:
total:4100488
used:3969352
free:131136
shared:0
buffers:68720
cached:3050716
Mem:
-/+ buffers/cache: 849916 3250572
Swap: 2618584 372 2618212
Most(if not all) modern operating systems try to utilize as much memory it is possible because it is better than sitting idle. Also it is not that easy to identify the true free memory on linux. The free command for example will not show you what you are looking for. Search google about identifying the true free/utilized memory on linux.
The “top” command is a lot better when you feel your system crawling and you need to find the program that is causing it.
In other words if your system seems responsive you shouldn’t really care about the free memory