6950 are outdated and they are 300-400$ for the both. You may get the 7950 for that price or even the 7970 which comes with 3 games with a total value of 150$ making the card worth even less, and giving your little brother some cool games.
if you're only using a single 1080p monitor go with a single 7950/7970 as suggested by lores97. heck, even a 7870's a good improvement over that 545, if you wanna save up and use the funds elsewhere, say on an SSD. either way, you don't need 2 cards unless you're running 3x1080p monitors or a single 1440p monitor that you want to get 45-60fps in most games at ultra.