we also use Dell 2850’s, and other than some off and on stability issues with the TE410P hardware, they’ve been largely reliable.
one thing though - since asterisk doesn’t use much HD space, i’d consider running a raid 1 mirror with a hotspare - that way you only use three drives per server and you’d actually have to lose all three drives for there to be any data loss…we were runnign raid 5 until one of the drives failed and the hotspare didn’t kick in. asterisk immediately locked up and we ended up losing the entire array. with raid 1, everything would have kept on chugging (and we even tested this by pulling 2 of the 3 drives - everything stayed up, although the server started throwing out alarms left and right).
i’ve also heard good things about HP servers - they seem to be fairly popular as well.