Hmm, not too sure about WOW, but Halo shouldn't require anything fancy. I bet a 9800 Pro would work excellently for him, and I've seen those go below 100$ shipped for a used one. That shouldn't damage his budget too badly. Hell, a 9800 Pro should be able to handle most games that are out right now pretty decently. I'd say to get the following:
CPU: AMD Athlon 64 Venice 3000+
mobo: either Abit AV8 or MSI NEO2 Platinum or the EpoX one, which ever is the cheapest/has the best rebate at the moment, even though I'm pretty sure the Neo 2 Platinum overclocks best.
video card: ATi Radeon 9800 Pro 128mb, but if he can afford it, a 6800 regular (nonultra/nongt) would be ideal. You can unlock the 16 pipes without artifacts, and it works quite nicely from everyone I've heard. Either that, or a X800 Pro Vivo version that you can unlock to a X800XTPE 256MB. A 6600GT would be fine, but the aforementioned video cards just own it. 9800 pro would be around 100$ shipped used, x800 pro would be around...eh, no idea tbh, but probably too expensive. as for the 6800, I've seen them at 130$ shipped used. Extremely good deal.
ram: 1gb corsair valueram pc3200 ddr
So far that's: CPU~146$, MoBo~95$, Video~100$-150$, RAM~89$
That's between 430$ and 480$ altogether...and he could probably squeeze in a dvd drive if he got the 6800, and just wear headphones instead of speakers like a true gamer should.