You've got really good taste.
However, the Maximus II Formula does NOT support SLI, so if you want to run 2 video cards, they must be ATI (unfortunately). If you want to keep it to 1 card, you could always look at the GTX 295...
And to give you an idea (for one-upping your brother more than anything else), the 4850 is equivalent to the 9800GTX+, so you would need at least the 4870 (equivalent to the GTX 260 Core 216) to meet your objective...
ATI is supposed to be releasing its next video card in April, if you can wait. The 4890, I believe, not a 5870.
That 9800GTX+ is a great card, especially when SLI'd. You might be charitable and build your brother a rig that would support that feature in the future (of course by then, you will have 2 4870s in Crossfire, so you're not worried).
Asus video cards are great, but with their fancy fans, be sure to double-check the widths of the cards. When you Crossfire, if your mobo doesn't leave a gap between the video cards, then they may not fit. Both of your Matrix cards are OK in this regard, but the Asus 4870x2 with the 3 fans has this problem. The Maximus II Formula also leaves room between the Crossfired video cards, so you're OK.
+1 on dual slot cooling with rear exhaust. Look into the XFX 4870 and 4850 cards, which have the more traditional rear exhaust design.
I recommended Sapphire up front because they are usually great value (although the 4850 doesn't have rear exhaust cooling).
Good luck!
P.S. The Xigmatek HDT-S1283 heatsink is the one you want.