If you have the money for a full $60 dollar games, I'd say get Dragon Age, but if you had the $60, you could also use it to get all of Mass Effect 1, Fallout 3 (non-GOTY) and TES4:Oblivion.
So I'm assuming you only have $20. I which case I'd say it depends on what you want. To expand on that ME2 experience, with great story, great atmosphere, and great characters/dialogue, but only about 20-30 hours, get ME1. For sheer length, solid gameplay, and a good story, get Oblivion. For a brilliant attmosphere, as well as fun gameplay, and quite good longevity, but a simply "okay" story, get Fallout 3.