Can it play some modern games... Yes! Manage them... Maybe! It will really depend on the game he plays and how well he'll be able to play the game on the PC... Example: He will have a very hard time with GTA but could manage COD4 at low levels.
Your CPU will be the weakest link in this. So OC the crap out of it if you can. Cut the OS processes so its running lean, and for modern, I'd say you'd be limited to the DX9 engines. The valve games should do okay, not great, but okay.
indeed overclocking the CPU will help. Overclocking the GPU a little will help too. The 4650 seems to be limited by it's memory bandwidth more often then not though. Anyway it should play slightly older games at medium or low-med. For Newer games it just depends on the game. I wouldn't recommend trying to play Crysis on it even at low settings in DX9.
By the way a friend of mine has an Athlon 64 +3200 with a Radeon 4350 and he plays alot of MMOs like Guild Wars and stuff so it seems fine for that. It was a huge upgrade from is 2.7GHz celeron . He likes to play alot of those free MMOs and it seems fine for that so if that's what your brother is gonna do you should be good. You may wanna add another stick of RAM there.
Actually, I know it'll play Crysis on low, 'cause I did with an AMD 3000+ (S754) and an X800GTO, both of which are far weaker than that system. So it should play pretty much anything, though your settings might be on or near low.