Speaking from my personal experience, I just recently flashed a spare Linksys WRT54GS router I had laying around with DD-WRT firmware, and after some very extensive tweaking managed enable it as a wireless repeater. This I would not recommend to anyone who doesn't have alot of time to burn. You'd save alot of hassle by buying a dedicated unit built for the job.
Maybe something like this would fit the bill:
Is your wireless network wireless-G or N, as the previous unit would only support G speeds?