And there is ALREADY a big difference in graphics that isn't going to change, both ends will improve over time.
The Wii's graphical ability is already at the top end. It's improvement is going to be incremental. Also, factor in that third parties don't even seem to care about producing adequate graphics and are still being rewarded with decent sales .. and it's not going to get much better than it is. The PS3/360 will see significant improvements, akin to what we've seen in the past generation from the first year until the years 3/4.
The graphical disparity will loom even larger when the price points are within shouting distance of each other. The X360 priced in the 249-299 range will absolutely mop up the Wii priced at 149-199. I don't even see how it will be close.
The Wii's sales lead is more a reflection of it's price and novelty than it's games. Once the price and novelty is negated, it will be about the games and X360 already has it beat.
Wii has plenty of room to grow, hardly anyone bothered to push the GC, and when they did they got amazing results, the hardware itself is plenty powerful to push a great looking game @ 480p
Not all shovelware is selling and yes some of it selling because of the "newness" of the control and that will die down and devs will have to make an effort. EA so far has done a pretty good job, but everyone can do better. If games can hang around prime/galaxy levels the graphics won't be much of an issue, unless you are hardcore gamer.
Pricing would be a factor, if anyobne outside the US really gave a rat's ass about the 360, it'll be fight in the states, but in other countries not so much, esp not in Japan.