If you have a brand new domain (i.e., you just registered it and it's "create date" is very new), then you'll want to buy an expired domain that has some history behind it...and a create date that's at least one year old. Redirecting an old expired domain will give your brand new domain a lot of benefits, especially when it comes to search engine rankings.

How do I find a quality expired domain?

I actually like the free service called Fresh Drop. You can search through the various auction listings like Godaddy's auctions and others. Sorting is pretty easy, I personally like to pick up domains that have .GOV backlinks (you can sort the domains and see which ones have .GOV links) and I also like to pick up domains that are currently in the Yahoo Directory (can help you save the $299 submission/review fee if you redirect the domain to your current domain).

If you find a domain that's not necessarily the "best sounding domain" or if it's not a keyword rich domain then using a 301 redirect from the expired domain you buy to your new or current domain will help with traffic and rankings.