If you look on Ebay, there's a guy that sells pre-modified Linksys WRT54G-TM (Tmobile) routers with DD-WRT firmware. His modified versions are very pricey, but worth every penny. I think I paid about $200 for mine.
His work is excellent (coming from a electronics rework tech, that says a bit)...he added additional heatsinking to the inside, overclocked the router's CPU and added extra flash memory.
It was doing just fine when I had a 30MBps down and a 12MBps upstream connection...never has a glitch (no power cycling under continuous high load usage, etc)...and the configuration options of the DD-WRT software is incredible from a security standpoint.