You could sub in the GTX 460 1GB (NOT THE ONES WITH THE SE ANNOTATION), however that'd be kind of meh, because you get worse performance for the same price. http://www.anandtech.com/bench/Product/291?vs=313
Almost the same, but on avg the 6850 wins by 5%.
One more quick question -- and sorry if this is redundant but I want to make absolutely sure.
Can I substitute the graphics card on the $792 build -- GIGABYTE GV-N460OC-1GI GeForce GTX 460 -- in to the $662 build (Replacing the XFX HD-685X-ZDFC Radeon HD) or do I need to get the higher-end motherboard?
It's not quite as simple as all the other forumers dismissing that guy -- if you read the whole thread, some others had similar problems (mostly with 4000 and 5000 series, but I'd rather be sure).
Honestly, I don't care at all about a 5% speed difference. But you mentioned crossfire -- I have no idea what that is. Should I be concerned about the geforce's effect on crossfire for currently-out games?
Crossfire is the dual card configuration for AMD cards. SLI is the dual card configuration for Nvidia cards. Crossfire and SLI would have a HUGE effect on current games if you ever chose to buy another (460 or 6850) card. Granted your only playing Cities in Motion ATM, but later on the CF/SLI would be beneficial.
As for the solution to the problems, upgraded drivers fixes the issue. As you can see in the link I posted above. If you really aren't sure though, just get an Nvidia card.