I know the CPU discussion here is mostly resolved, but I thought it'll be worth throwing in a couple of points to consider, especially if you plan on upgrading it sometime soon:
There's seemingly a lot of controversy about this topic really and I've seen arguments for all three architectures. From what I've seen however, this seems to be
reasonably accurate in terms of results:
http://www.tomshardware.com/reviews/gaming-processor-frame-rate-performance,3427-9.html
Trouble is, CPU performance tends to differ from game to game, depending on the optimizations and how intensive it is. With the original topic of bottlenecking, you'll be perfectly fine with most singleplayer games, in which case you tend to be heavily CPU-bottlenecked. With such applications, you'll find that a HD7950 may not be bottlenecked at all, because of how the game is designed.
With more CPU-intensive games and multiplier situations however, there's a lot more stress on the CPU, so you'll probably run into a bottleneck. This is also where some of the disagreements start, some games can utilize multiple cores (seemingly in multiplayer especially), however others do not at all. With more lightly-threaded games the i3 pulls ahead, but the Phenoms and Vishera counterparts tend to perform better with better threaded titles - which is why I said the article above is only reasonably accurate.
Personally, I'd go with a HD7870/7950, depending on what titles he plays. Phenoms are still pretty fast, clock to clock they are very similar to Vishera, so your current 965 is already pretty similar to a stock 4300/6300 in terms of performance.
On the note of your own rig, it may be worth considering getting a 7950 for yourself and then give your brother the 670. Might not be ideal for you to have an AMD card in your system, but both perform similarly anyways so it's a pretty fair trade. With mutli-monitor setups however, it'll give the option to add in a second card without running into VRAM bottlenecks.