OS system requirements haven't really changed all that much. Don't invest too much time worrying about the OS, as I'm sure pretty much anything suggested here would do just fine (that said, I'll throw my personal vote for Xubuntu), and that any responsiveness differences between them would be negligible, if even detectable.
What you should spend your time worrying about is the stuff that runs on top of the OS.
For example, I suspect you'll want to browse the web, right? Yesterday, I was at a news site (The Guardian) and I wanted to view some multimedia on it. My default browser, Firefox, runs with NoScript, so the stuff wasn't working. Instead of going through the trouble of whitelisting stuff on the site, I just loaded the page up in IE. Between the copious amounts of JavaScript and flash video ads and other crap, IE--which I had just started fresh and had only that tab open--ballooned to about 800MB and was noticeably lagging (this is a netbook). I also tried it in Chrome, and it was no better. And it's moments like this that I'm reminded of why I use Firefox with NoScript. I tell you, the modern web has grown out of control--especially since lazy developers these days, instead of coding from scratch, use huge JS libraries--sometimes over a megabyte large for even simple tasks (a whole effing MB of interpreted code?!?! are you bloody serious?!).
The OS isn't your enemy. The stuff that run atop it is. If you have a low-end system, spend your efforts in controlling that instead because, seriously, pretty much any distro would work just fine for you.