Relations between the two are currently fairly good, and war between the two countries is seemingly unlikely. That being said, the two have had their share of territorial disputes, and at this point it is probably not a good idea to consider any dispute with China dormant.
Both Russia and China would have their own advantages and disadvantages in a war. The Russian military has gone to war numerous times in the last twenty years, in Chechnya, Georgia and now Ukraine. Russian forces, although often ill-trained and ill-prepared, are resilient and capable. Russia also holds a technological edge over China—for now, anyway.
The flip side is that Russian equipment, on balance, is fairly old. Russia’s economy is also less than a fifth of China’s, which will chip away at her strategic position. Another issue is as old as Russian power: much of the country’s military lies west of the Urals and would need to be sent east by air and rail.
China’s main advantage would be that the bulk of the People’s Liberation Army would be relatively close to the theater of operations. The PLA is also larger than the Russian military with fewer distractions—such as NATO or a restive Caucasus—that require an armed presence.