Any of the three systems in addition to the PS2 are readily available here, and surprisingly the prices, despite being imported, are roughly the same you'd pay in the States. A recent inquiry at my local dealer said the PS3 is going for about 2700RMB (~$400), an Xbox360 Arcade is 1600RMB (~$220) and the Wii runs somewhere around $300 last I checked, but that was awhile ago.
Where to buy:
In Beijing there are two choices--big malls like in Zhongguancun, the electronics/tech gadgets lala land of Beijing, where huge multistory complexes exist that cram in about as many vendor stalls as possible--if there were a fire, I'm sure most people would not get out alive. That said, it's the easiest place to find just about whatever you want, including video games and their systems. Zhongguancun is too far for me, but there's a similar place on a smaller scale near where I live called Pingleyuan. It's a lot of the same stuff but much quieter, and the guy who sells systems there is cool and seems pretty knowledgeable about gaming. The other choice is the little shops around the drum tower area--for some reason that's a popular location for console selling.
I bought a white PS3 last June, 40GB model, and it was by and large a hassle-free experience. Generally you don't need to bargain much, but you can probably knock 100-200rmb off the price if you get games and accessories with it. Most shops will have plenty of accessories, especially important cables like HDMI and even HDMI to DVI cables for computer monitor users. As far as I can tell they do come with a warranty, but I'm not sure it's the manufacturer's--rather you can either return it for a refund, get it exchanged, or get it repaired depending on how many days have passed.
There are some things to remember, however--the most important, I think, is that pretty much any Xbox360, Wii, or PS2 you buy will have been modified to play pirated games. This is done for the obvious reason--cost. You get a modded console and you can buy games just like buying pirated dvds--something like a buck a pop. The downside to this, at least for the Xbox360, is that you cannot play online.
The PS3, however, has not been modded. I guess no one's figured out how, or maybe the infrastructure doesn't exist yet to pirate Blu-ray discs in mass quantities for cheap. This means all the games you buy will cost you their actual price, which is anywhere from 260-400rmb. I find that there aren't any bargain deal prices like you can find in the States on older games, and I've never heard of used games being sold here either. The plus side is, you can play online with the PS3.
Other important considerations: remember that all consoles are imported, and if you like to watch DVD you bring from home, you might want to reconsider buying one here rather than one from the US, as consoles are region locked and in China, all consoles come from either Japan, Korea, Hong Kong, or maybe Taiwan. This means no legit DVDs from the west will play in them. Lamer than a paraplegic chicken, I know. Even worse, the PS3s are all NTSC and won't play PAL anything. Yet oddly when I rip a PAL movie to an avi file, it works fine. Dunno what's up with that.
OH wait, the actual worst part is buying a Wii here, because you can't change the damn language settings. If it came from Japan, you're stuck with Japanese, which I imagine has caused many hurt Chinese feelings.
Finally, the Xboxes, depending where it was imported from, may have 110v only power supplies, in which case you'll need a power invertor or power regulator to convert the voltage--DO NOT use one of those little crappy Radio Shack ones. It will fail and then you'll cry.
Other links on buying video game systems in China:
And in Taiwan:
The latter raises an interesting point--that the Wii doesn't even offer Chinese, simplified or traditional, as an option, suggesting that Nintendo doesn't care much for Chinese-speaking markets. I'm sure that would change the microsecond China starts allowing consoles to be sold on the mainland.