Techbargains has a great list of laptop sales: http://www.techbargains.com/catsearch.cfm/0_3_0
Keep an eye on that list and bit when you see one at the right price. All that matters is your CPU, if you want to keep it simple. If you spend $499, you should get an i3-2310M. If you spend $400, try to get a P6100 or so.
This is a decent CPU ranking: http://cpubenchmark.net/
Compare everything to where a T4500 ranks. It's the minimum CPU I'd go for if I were you. I don't know about Athlon mobile CPU names too much. Check their ranking before you buy any.