I don't get it, are you building him a new system or just upgrading the graphics card?
HD 6450 is way more powerful than 8300 GS, however, it's poor buy for your money and it's also very slow for games of today. I'd recommend at least HD 6670. His CPU should be OK with it. Even if it bottlenecks a bit, it will be a way better buy than HD 6450, since it performs way better.