- Chipset manufacturer (Broadcom, Atheros, etc). Faster is better as this is the processor that needs to decrypt and encrypt the VPN tunnel. Some chip sets are simply not compatible with DD-WRT.
- Flash size (2 MB, 4 MB, 8 MB, etc) not to be confused with RAM. This is the amount of space available for DD-WRT to use. More space is better. Flash size dictates which features you can install. To install OpenVPN you need at least 4 MB of Flash with a Broadcom processor. 8 MB is required for all other processors. For this reason many people favor Broadcom as a cost effective solution. 8 MB with a Broadcom will allow you to install the full version of DD-WRT with all its features. Flash above 8 MB allows you more space to compile your own utilities for a full Linux experience.
- RAM size (8 MB, 16 MB, etc) is the working memory of the router. It is almost always larger than the available Flash if the information you get is poorly labelled. Once again more is better especially when running a VPN.
The grand daddy of DD-WRT compatible routers is the ASUS RT-N16. It is a Broadcom based router with clock speed of 533 MHz, 32 MB of flash and 128 MB of RAM. It offers 300 Mbps wireless service, 1,000 Mbps ethernet service, 300,000 sessions and 2 USB ports for external hard drives, printers and the like.
To sum it up if you’re buying a new router for DD-WRT: 8 MB Flash or more, a faster processor is better and more RAM is better.