One caveat to this is that Apple frequently puts something at or near bottom of a range, particularly a new range, that is especially crippled. The first instance of this (Mac LC) is old enough to vote. Check out
http://lowendmac.com/roadapples/worst.html to get a feel for it.
Things to look for in the order I personally prefer: maximum RAM capacity (prefer high), processor cache size (prefer large, generally), processor multiplier (prefer low), RAM speed (prefer high), processor clock speed (prefer high)
If you don't want to pour over Intel CPU specs, I'd just pick something in the 35th percentile of price and performance. Don't worry about amount of RAM pre-installed or any parameters of the hard drive. If the computer begins to run slow, pay a buddy to find a deal on Mac compatible ram and buy the largest amount your system will use. Keep regular backups. When the hard drive dies pay the same buddy to research the largest and fastest disk that uses less than or equal to the number of watts your current disk does (thermal issues make higher power/heat drives risky).
This is slightly simplified but should give you a system that has the most upgrade potential to protect your investment.