Same here...

I deem it's the amount of RAM on your machine.

but it used to work kind of ok till I installed R16...
And I have a 4 GB RAM installed..... is that not enough?

Do you have latest graphic drivers installed?

>And I have a 4 GB RAM installed..... is that not enough?
It depends on the OS you use.
For Windows XP, you must have the 3Gb patch.

The other possibility is indeed a incompatibility with your graphic card driver.

Regards,
Fabien

If memory was an issue, try to minimize the use of Virtual Memory on the cnext.exe process (VM Size).
Can you open the assembly in Cache mode, and load only the models that are needed in design mode?

And the "Latest" graphic driver is not always the best solution.  Search thru the Dassault Certified Hardware list to find the Certified graphic driver and you will have much better luck.

V5 R16 has a 32 bit and a 64 bit version.My company has loaded Windows XP 64 bit and the 64 bit R16 into our Dell XEON PCs. We have completely eliminated this issue.

The 32 bit version (for some reason) will only utilize 2GB of RAM then crash. It does not recognize that you have loaded more.
If you are unable to step up to a 64 bit version, then I would recommend loading and unloading files in your large assemblies. This will help with conserving memory.

good luck

You can use more than 2Gb of Virtual Memory under Windows XP ; you can patch it.
For more information: http://www.gehrytechnologies.com/catia/catia/catia_windows_large_memory.htm

Regards,
Fabien

Lets explain the whole memory thing on windows.

All processes (aka program) are allocated 4gb of address space   2^32 by the windows memory manager. This is regardless of the physical memory installed in the workstation it will use the page file to provide any additional memory. This address space is split 50:50 Process & kernel. Therefore each process has a maximum size of 2gb, this is a fixed limit on windows 2000 but on windows XP the ratio of process to kernel can be changed to 75:25 giving the process a maximum size of 3gb. For an application to be able to use more than 2gb of memory it needs to be large address space aware, R15 & R16 (and possibly R14 I can never remember when it changed) are. An application can be modified to be large address space  aware using imagecfg or editbin. The use of the 3GB switch is detaied in numerous documents, one is mentioned above but there are others available (find the IBM PLM support web site).
If more memory is required then a 32bit large address aware catia can be run on WinXP64 this will allow the CNEXT process to use 4gb of memory. If this is still not enough then using R16 or higher 64bit version on WinXP64 will allow the cnext process to use 8gb of memory. The theoretical memory limit for a 64bit os is 16tb (2^64) but at the moment WinXP64 has a limit of 16gb (it many $$$$ to get a machine with this much memory) which is split 50:50 process to kernel. Who knows there may be a 12gb switch for XP64 or you may just move to vista or what ever follows.

There are also many tools in catia to reduce the memory consumption when handling large assemblies. Look up working with cache, managing representations, selective loading.