I currently use my Samsung PN51E550 for gaming. I primarily use my Xbox 360 on it, but it does get some PC game time in as well. Mind you, it does feature active 3D, but you don't have to use it if you don't want to. Keep in mind that active 3D is far superior to passive in terms of picture quality and 3D effect. I find the 3D to be very good with some tweaking, and I'd be happy to share my settings should you decide on this set. The TV retails for around $850 USD. I've been playing loads of Halo 4 on it, and as long as I set the screen wipe feature on it (or just watch TV) for an hour or so after my long gaming bouts, I don't see any IR on a white screen (Halo 4 DOES implement a HUD that has slight transparency, which helps with that). That being said, games with more prominent HUD's, such as Battlefield 3, have caused IR, but, like I said, the issue can be negated long as you put on some other fullscreen content when you aren't gaming. I really can't stress that enough if you want to use a plasma for gaming and don't want to see any burn-in.
As far as input lag goes, I don't even have game mode enabled, and I don't have any problems with it. I have all my games passing through the HDMI receiver below as well, with no noticeable lag. On the TV's default settings, I've played Rock Band and DJ Hero on it without any need for calibration. I see games like that as the ultimate gaming input lag test, and, in my opinion, the TV passes with flying colors (no pun intended!).
In regards to an HDMI receiver, I purchased an Onkyo HTiaB from Amazon a while back for under $300. It's basically an entry-level setup for a small to mid-sized room, and includes pretty decent speakers and a subwoofer. I use it in my bedroom, and love it. It also features 3D passthrough, so it's compatible with anything you throw at it. Take a look: http://www.amazon.com/Onkyo-HT-S3500-5-1-Channel-Theater-Receiver/dp/B0077V88V8/ref=dp_ob_title_ce
It's obviously not the best you can get, but offers a good bang for your buck. Not really recommended for use in a large room, as the speakers are fairly small and it only pushes 110W, though it does get pretty damn loud. Be prepared to buy speaker wire if you go with this, though.
As other users have said in this thread, you may want to consider an LCD or LED. I do believe that plasma has the superior picture for gaming, as LCD/LED's have motion blur. Plasma doesn't, which is a huge deal for me in terms of gaming. However, plasma TV's require an extra amount of care from gamers in order to avoid ruining a set with burn-in from constant and repeated gaming sessions. If you want a TV that you can just play the hell out of games on and never have to think twice about it, perhaps an LCD or LED is the way to go, especially if you're considering playing a lot of 4:3 content on it.