There are probably 2 ways one can go to get signifacantly more power than a NSLU has. BOTH are considerably more expansive however:
1. Synology DS107+ mit 128 MB ram. Definitly a step up in speed but also in price and the “hacking” support is present but limited. Its expanding though.
2. Building a low energy Mini-ATx computer. That is a smaller formfactor than the usual ATX used for normal computers and there are highly integrated motherboards with onboard processor available. There are 1 GHz solutions with average. 30W consumption only. But this is a lot more expansive than the NSLU.