Maybe the Buffalo WZR-HP-G300NH? I've used the non-gigabit version and it seemed fast and stable:http://www.newegg.com/Product/Product.a ... -_-Product
It *ships* with DD-WRT (a custom version by Buffalo, but it works) and has a fast processor in it.
The Asus router previously mentioned has a pretty good reputation (not sure about long-term reliability), but I've never used one. Specs are good, though.
Also, I haven't looked at any benchmarks at all, but I would think a sub-$100 consumer router might have trouble pushing a 50Mbit WAN connection. But, I guess you never know these days.
I also have the Netgear WNDR3700-100NAS, and I've been very happy with it (fast AND stable), but as far as I know it doesn't run 3rd party firmware, and it's well over $100, so it's probably out for you. But, I really like mine, so I always toss it in as a suggestion.