First you need to update the firmware with the 'mini' version of dd-wrt specifically for wrt350n. Then you can install the 'generic' version (std, voip, vpn whichever you want) of dd-wrt. Be sure to reset to factory defaults before and after each flash. You need to do the 'mini wrt350n' first because that is the only one the original Linksys firmware will let you upgrade to.
First of all, what version router do you have (check the bottom)? Only the version 1 will work with DD-WRT. _________________ WRT54G v2.0 (Bridge)
WRT54GL v1.0 (Bridge) (x3)
WRT54GS v3.0 (Routing)
WL520gU (Routing)
Running DD-WRT v24 SP2 - http://www.dd-wrt.com
After that you should be able to flash it with dd-wrt.v24_std_generic.bin
Be sure to reset to defaults afterwards. _________________ WRT54G v2.0 (Bridge)
WRT54GL v1.0 (Bridge) (x3)
WRT54GS v3.0 (Routing)
WL520gU (Routing)
Running DD-WRT v24 SP2 - http://www.dd-wrt.com