[CentOS] Problem to run 4 virtual machines at a time.

Hi all

I have installed  vyatta system, (vyatta-livecd-vc4-alpha2.iso ) on top of
Centos5.
Our system requirement is,
* Centos 5.1 (2.6.18-92.1.10.el5xen )
* 2 Gb RAM,
* x86_64

And cpu information is as follow,

[root at turtle4 ~]# cat /proc/cpuinfo
processor       : 0
vendor_id       : AuthenticAMD
cpu family      : 15
model           : 107
model name      : AMD Athlon(tm) 64 X2 Dual Core Processor 4000+
stepping        : 1
cpu MHz         : 2100.010
cache size      : 512 KB
physical id     : 0
siblings        : 1
core id         : 0
cpu cores       : 1
fpu             : yes
fpu_exception   : yes
cpuid level     : 1
wp              : yes
flags           : fpu tsc msr pae mce cx8 apic mtrr mca cmov pat pse36
clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt lm 3dnowext 3dnow
pni cx16 lahf_lm cmp_legacy svm cr8_legacy
bogomips        : 5252.03
TLB size        : 1024 4K pages
clflush size    : 64
cache_alignment : 64
address sizes   : 40 bits physical, 48 bits virtual
power management: ts fid vid ttp tm stc [6]

processor       : 1
vendor_id       : AuthenticAMD
cpu family      : 15
model           : 107
model name      : AMD Athlon(tm) 64 X2 Dual Core Processor 4000+
stepping        : 1
cpu MHz         : 2100.010
cache size      : 512 KB
physical id     : 1
siblings        : 1
core id         : 0
cpu cores       : 1
fpu             : yes
fpu_exception   : yes
cpuid level     : 1
wp              : yes
flags           : fpu tsc msr pae mce cx8 apic mtrr mca cmov pat pse36
clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt lm 3dnowext 3dnow
pni cx16 lahf_lm cmp_legacy svm cr8_legacy
bogomips        : 5252.03
TLB size        : 1024 4K pages
clflush size    : 64
cache_alignment : 64
address sizes   : 40 bits physical, 48 bits virtual
power management: ts fid vid ttp tm stc [6]

My problem is  i have installed  '4'  virtual machines.
each virtual machine have 256Mb RAM.
But i am not able to run 4 virtual machines at a time.
 Please help me, what should i have to do?

Thanks
mallu
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