A while back I bought a fairly cheap Garmin Nüvi 300 from a nice eBay seller who'd taken the trouble to make sure the firmware and map data were up to date, and even pointed me towards somewhere to get free POI and speed camera data. I prefer trying to drive in compliance with speed limits, personally, but the thought was appreciated.
When you Google for information on the 300, and the 3xx range in general, you may find the information out there on it to be a little bit confusing. Some reviews and listings mention that it can handle TTS (text-to-speech) output, which is necessary in order for the unit to speak roads rather than just give confusing left/right directions. In actual fact it appears Garmin intended only the 350 and up to have this capability, and went so far as to backtrack and remove the function from newer 300 units. Say it with me: bastards.
If you're prepared to revert the firmware (and rest assured this is easy, since it's just files stored on normal flash memory) you can dial it back to version 2.60 or earlier — version 2.40 of the firmware is available here. You might want to change the background image in your unit's non-accessible memory before reverting, as the ability to do this was added in a later firmware.
You then need to get a TTS voice or two. Having spent some time messing around with these for dyslexic friends, I've found Karen (Australian English) to be very clear and not too artificial; the woman who recorded it is a pro singer, in fact. You can't download TTS voices directly to the satnav since Garmin are, as indicated previously, bastards. However, it's available here and all you need to do is extract the archive, strip the first 60 bytes from the .rgn file (which you can do with XVI32 or similar) and rename it with a .vpm extension. Then put it into the voices directory on your unit. People who like meddling even more can change the phrases Karen speaks using this piece of free software. Information for this series of tweaks came from here and here — a big thanks to those people.