Your best bet, if you want a zero-cash-cost solution, is to hope that there is a current version of DD-WRT available for the device (meaning a current Linux kernel that is still maintained, and current firmware). "Current" in the world of OS security likely means in the past 6-12 months, at most.
There are many more-powerful devices available, starting at under US$20, that meet the current recommendations of
- At least 16 MB of flash
- At least 128 MB of RAM
- Wireless chips that support current 802.11 specs
(older specs, like 802.11b/g, can dramatically limit throughput, as well as being insecure)
One place to look for suggestions and opinions is the very long thread