Funny you should ask. I just bought a new wireless N router (Linksys WRT350N. I picked it because it has an 8MB of flash memory so it can take the largest flash of DD-WRT.
Mine required me to install the mini flash first, and then the mega flash. You are lucky someone installed it for you because it turns a $100 router into a $600-$700 router.
mbpark is correct when he says you should watch out for overheating. I increased the power to my antennas but I made sure not to overdo it. The range on this thing is ridiculous. I can do QoS, I can setup layer 3 VLANs, I have a huge number of features that aren't normally available on this router.
You can check which version you need based on your hardware model and version number.
http://dd-wrt.com/wiki/index.php/Supported_Devices