I was able to do a 6 week trip around South East Asia with this bag...
Columbia Silver Ridge Weekender (I got it for 79$ at Century 21 in NYC) While smaller than most bags, it did the job.
Another tip... Space bags... those things are a life saver, you can fit in tons of clothing in a limited space. Also they allow you to keep things dry... the bad thing is that since it is a grey color, it attracted tons of stains/dirt while on planes...