In the "Wireless" section, under Advanced, you could make sure you are using the full bandwidth of wireless N (making it use 40Mhz, instead of just 20Mhz. Your neighbors might hate you for it though

)
Wireless-N Configuration - DD-WRT Wiki
Overclocking the CPU won't give you much more throughput, unless you are torrenting (uses a LOT of CPU). You could on the other hand try to bump up the mW on the output power. Use that with care too, since just like overclocking, you could be shortening your device's life span. The design of your router is fairly bad for overclocking/power increasing. It's already got heat issues:
Linksys WRT310N Wireless-N Gigabit Router Review - Watch CNET's Video Review
Also, keep in mind that location, location, location is very important. You can't change the antenna, so you need to keep it in a good spot.