well he won't experience much lag based on when i play it if i compared it to this other system of mine after doing some check.
i7 3930k, 7970 in xfire, compared to this ones
i5 2500k, gtx560ti 2gb in sli
phenomii x4 gtx 560ti 2gb in sli
phenomii x6 1090t hd 6970 in xfire
i3 2100 hd 6870 xfire
all of these are at 1920x1080, 1440x900, 800x600
also tried it with my dell studio hybrid (this one caused me some lag)
all settings on high, and based so far on multiple test, it's either i didn't see MS on my 7970 compared to my 560ti's, hd 6970/6870,
and the only overclocking i did would be my cpu, just to match some of them to the gpu im using, although those MS doesn't really affect much gameplay well at least those on my level which is at average or below average at cs 1.6
oh and my dell studio hybrid plays on vista 32bit compared to the other 64bit
but im sure he can live with some core2duo as it works nicely on my dell studio hybrid and that using some built in graphics, so if he were to use something like a gt 210 or a 8800/9800 it should even be more than enough of what he need and some nice 2.7ghz-3.0ghz speed for whatever case might happen, some good internet speed and of course using a single card just to prevent ms, and well for a 120hz monitor i don't know much since i don't own one but if you need a more powerful card take a look at gtx460 or those hd 5xxx should help him out, and even 2gb-4gb ram is more than enough,