Class A DP 1:
It is not too hard to install. Just a few words of caution:
1) you should be fairly comfortable with computers (but don't need to be a guru)
2) the Linksys WRT54G is probably the best option as it has more ram than most (even new routers) and in its day it was one of the most common, making it easy to find on eBay
3) make sure you follow the upgrade instructions to the letter and there are a number of versions of wrt54g (about 8 different versions)) so make sure you follow the instructions for the right version
Here are the instructions for installing DD-WRT:
http://www.dd-wrt.com/wiki/index.php/Linksys_WRT54G/GL/GS/GX
Here are the instructions for configuring for the RV:
http://www.dd-wrt.com/wiki/index.php/Universal_Wireless_Repeater