I have the Mugen 3800 for my X5 and it's been great. Obviously that won't fit in the case (unless you have the extended battery version) but I think you won't go wrong getting their 2000. I've considered it myself.
And just one more note on real-time strategy games, if you want a fun little cheap one, you might want to give
Sploit! a try. I had a lot of fun with it and it's only $2.95. There's a trial with the first few levels if you want to check it out before you plunk down the cash.