Perhaps you could take into consideration the chances that your friend will want to upgrade the system in the future. Especially with the way games are improving these days! Sometimes spending the bit more cash at the start can save a lot of money later!
If this was my build i would opt for a motherboard that supports Xfire and a more powerful PSU. This way if i ever found my system struggling with graphic intensive applications like games, i could add another graphics card with no trouble!
The Xfire scaling for HD6870s is very nice!