First time poster, long time lurker (sorry about my wallet already, and getting sorrier all the time!) and I've recently dealt with this very thing.
I know the RCA-to-an-adapter route is the cheapest, but for me it sounded really bad. Like, not workable.
I tried running it through the line-in on a Total Bithead, and it just sounded really noisy to me. (This was with ER-4Ps and with Sennheiser HD-580s.) Crackle, crackle, hiss, hiss. Also the volume dynamics were strange. Either blaringly loud and piercing or barely audible. I can only imagine that it would sound worse with a little volume controller.
Best solution I've found so far? The JVC SU-DH1 surround adapter (which I read about right here at head-fi). I got it from Audiocubes.com for about a hundred bucks. Couple of gotchas, but surmountable ones.
1.) You need to buy the Japanese version if you don't want to deal with batteries. (It's the one that Audiocubes sells.) The American version doesn't have an AC Adapter jack.
2.) You have to, you know, BUY the AC adapter. 10-15 bucks at Radio Shack, and you're all set, though.
3.) The optical in is a miniplug, so to connect it to the Xbox 360, you'll need an optical cable and a miniplug adapter. But they're, like, a dollar.
Honestly, this setup sounds fantastic to me for gaming. I find myself using the ER-4Ps with it (not wild about the Senns) and I am blown away at how full the sound is. And all for a little over a hundred bucks (plus the cost of whatever headphones you use.)
Not as cheap as the adapter option, of course. But not crazy-expensive either. Highly recommended, for what it's worth from someone with one post. Including, you know, this one.