A year past when this was originally asked and I still say Joyce beats Valuev. Joyce lost to a southpaw giant that was faster than him, Valuev may very well be the only boxer slower than Joyce. Also, Valuev despite his size wasnt really a hitter. Once he started fighting top 15-20ish guys his ko's pretty much disappeared and he was fighting guys who were of similar durability to what Joyce has shown. Joyce wins and extremely ugly UD
Joyce would outwork him to a decision. Valuev hasn't got enough power to stop him and despite his height wasn't hard to close down.
Valuev won't stop him, won't smother him, and he has no inside game to reasonably outpoint Joyce and smother him when it matters. He would get battered now and then, and it would add over time. EDIT: Funny how I went 180 here.