
enough said?
$130 will get you card that can run beautifully if you tweak some of the graphics settings (you really don't need to play max everything... the game barely looks any better). Heck, the GTX 460 768MB card is so cheap with MIRs, that you could splurge a bit if you wanted to... but you said cheapest, so find anything on par with the 550 Ti and get the cheapest (550 ti is about $130 atm). Everyone is making suggestions as if this is your main gaming computer.
I've played metro 2033 with an average of 45 ish fps and didn't notice any 'lag' (none of my other games will go below 60 fps if I don't crank up the AA). I'd say a minimum framerate of ~40 fps is a good place to have fluid gameplay and the 550 comes close to that at max WITH HBAO enabled.