linux cpu info and 2.6 kernel

Yesterday I upgraded a web server running Redhat Enterprise Linux AS 2.1 to AS 4. I did the upgrade because I had read that the 2.6 kernel was more optimal for high-traffic servers. and RHEL AS 4 supports the 2.6 kernel better than AS 2.1. Before the upgrade, "top" displayed four CPUs. Now "top" only displays one CPU. cat /proc/cpuinfo, though, lists four CPUs, as it did before the upgrade.
Two of the CPUs share the same core and physical ID, and the other two share another core and physical ID. I'm rather confused ... does this indicate that the server is a dual-core system? How many CPUs are there? What are the implications for hyperthreading? Does apache need to be compiled differently from the default to take advantage of hyperthreading?

I'm running kernel 2.6.9-34.ELsmp. The old 2.4 kernel was also SMP.

processor       : 0
vendor_id       : GenuineIntel
cpu family      : 15
model           : 2
model name      : Intel(R) Xeon(TM) CPU 2.40GHz
stepping        : 5
cpu MHz         : 2400.168
cache size      : 512 KB
physical id     : 0
siblings        : 2
core id         : 0
cpu cores       : 1
fdiv_bug        : no
hlt_bug         : no
f00f_bug        : no
coma_bug        : no
fpu             : yes
fpu_exception   : yes
cpuid level     : 2
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe cid xtpr
bogomips        : 4803.93

processor       : 1
vendor_id       : GenuineIntel
cpu family      : 15
model           : 2
model name      : Intel(R) Xeon(TM) CPU 2.40GHz
stepping        : 5
cpu MHz         : 2400.168
cache size      : 512 KB
physical id     : 0
siblings        : 2
core id         : 0
cpu cores       : 1
fdiv_bug        : no
hlt_bug         : no
f00f_bug        : no
coma_bug        : no
fpu             : yes
fpu_exception   : yes
cpuid level     : 2
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe cid xtpr
bogomips        : 4797.98

processor       : 2
vendor_id       : GenuineIntel
cpu family      : 15
model           : 2
model name      : Intel(R) Xeon(TM) CPU 2.40GHz
stepping        : 5
cpu MHz         : 2400.168
cache size      : 512 KB
physical id     : 3
siblings        : 2
core id         : 3
cpu cores       : 1
fdiv_bug        : no
hlt_bug         : no
f00f_bug        : no
coma_bug        : no
fpu             : yes
fpu_exception   : yes
cpuid level     : 2
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe cid xtpr
bogomips        : 4797.99

processor       : 3
vendor_id       : GenuineIntel
cpu family      : 15
model           : 2
model name      : Intel(R) Xeon(TM) CPU 2.40GHz
stepping        : 5
cpu MHz         : 2400.168
cache size      : 512 KB
physical id     : 3
siblings        : 2
core id         : 3
cpu cores       : 1
fdiv_bug        : no
hlt_bug         : no
f00f_bug        : no
coma_bug        : no
fpu             : yes
fpu_exception   : yes
cpuid level     : 2
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe cid xtpr
bogomips        : 4798.02
LVL 1
abaloniaAsked:
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ravenplCommented:
Yes, recent versions of top shows summary for all CPUs.
Toggle with pressing digit '1'

There are two CPUs with HT (which makes them four) (cpu cores       : 1)

> Does apache need to be compiled differently
No.

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