World of Warcraft is generally a very "easy" going game on hardware. However, it really depends what content your playing.
WOW's Cartoony graphics run just fine even on fairly low end graphics cards @ 1920x1200 resolution!. Used to play the game at that resolution using a Geforce 6800GT.
However, if your son is into "raiding content" WOW can get very demanding on the CPU.
I made a few tests at Kalecgos ( a dragon in Sunwell plateau). Due to the nature of the raid the tests arnt perfect, but should indicate what I mean.
All tests made with the same graphics settings except resolution, the PentiumD computer only having a 22inch monitor. Multisampling at X1, and Texture filtering 1 notch to the right. |--|--------|
Pentium D 805 (Dual core Pentium 4 @ 2.66Ghz) (resolution 1650x1080)
Geforce 7900GTX
FPS during boss fight 5-9
FPS looking at sky in Nagrand 100
Pentium D 805 1650x1080
Geforce 8800GTX
FPS during boss fight 5-9
FPS looking at sky in Nagrand 150
Core 2 E6700 @ 2.66Ghz 1920x1200
Geforce 7900GTX
FPS during boss fight 20-35
FPS looking at sky in Nagrand 180
Core 2 E6700 @ 2.66ghz 1920x1200
Geforce 8800GTX
FPS during boss fight 22-38
FPS looking at sky in Nagrand 250
Finally
Core 2 E6700 @ 3.2ghz 1920x1200
Geforce 8800GTX
FPS during boss fight 28-50
FPS looking at sky in Nagrand 255
Max fps was considerably better on the 8800GTX regardless of what computer it was installed in, but the minium fps are completely CPU bound, with the 7900GTX keeping up with the 8800GTX in most situations. The 8800GTX does allow higher AA/AF without hitting FPS too badly, while the 7900 suffers, but in terms of game playabililty for a "guild raider" cpu power is the main requirement imho.
For 5-10 person groups(and raids) then the computer he has is probably more than sufficient, but if its stuggling with frequent spells of minium FPS, than even if you changed the motherboard to get a PCIe slot and fitted the best graphics cards, it would hardly change it, as you'll get bottlenecked by the CPU.
Ask your son if he want to improve "raid play" or single group content. For single group a GPU will improve max FPS, and allow higher settings on eyecandy / resolution. For raiding, a high end CPU even coupled with a mid range GPU will perform a lot better than a low end CPU and a high end GPU.