My suggestion would be to spend more upgrading the derailers and not worry about the cranks. Hopefully someone will correct me if I'm wrong, but I don't think you'll see any difference other than maybe a few grams by upgrading the cranks to Alivio, but I think you'll notice a nice difference in performance by upgrading the rear derailer in particular as much as you can. If you could upgrade to 8/9speed Deore, for example, I think you'd have a much improved experience on the trail. Whether you can go 9sp would depend on your rear wheel(freehub). I'd bet you could upgrade the rear to 8/9sp Deore derailer, shifter, and cassette for under $150. It would require a new chain, though, so another $25+ there depending on how high you want to go on quality. Even if you stayed with the 7sp, though, I think it would be more of an upgrade to get nice derailers than to upgrade the cranks.
This is all if you ride trails with your bike. If you use it for commuting, etc, disreguard my suggestion, as it won't matter as much then for the shifting, but perhaps saving a little weight on the cranks or something might be more important. If that's the case, though, I'd still think there may be other more worthwhile upgrades. Perhaps some nice light weight commuter tires rather than the crank.