Yes, I can almost guarantee that a mobo that supports Crossfire/SLI will be able to handle the 460; however, I'm not sure exactly what the GTX460 requires, so I would double check. Essentially, PCI-E x16 slots are all the same. If that's all it requires, then the only other thing you have to check is if you can give it enough juice. This is what you should be careful about, because some cards (especially high-powered cards like this GTX series) require their own dedicated power cable. This is easy enough to do, unless you're running a dual-graphics card system. Make sure you get a PSU that has the cables you need. More on that in a minute.
Also, I noticed that you said "I looked into motherboards like Gigabyte's GA-890FXA-UD7 GA-890FXA-UD5
These are awesome.. but they use ATI cards..." You're probably talking about the onboard graphics chipset. If you're going to get a video card then the onboard chipset doesn't matter at all. Motherboards don't care whether your card is ATI or Nvidia. In fact, a few months back I decided I'd had enough of Nvidia and bought an equivalent ATI card. Uninstalled all the drivers, pulled the Nvidia out, put the ATI in, installed new drivers and it has run like a champ ever since. The only questions are: does your mobo have the right slot, and can you give it enough juice?
Also, near as I can tell, a motherboard is a motherboard. Really not much difference between them, except that you get what you pay for. That being said, I really don't know much about them. Just make sure it supports the hardware you want (enough PCI-E x16 slots, the right RAM speed, the right CPU socket, etc.) and has the features you want (onboard 5.1 surround sound, onboard wireless, enough USB slots, etc.). As for size, I would recommend an ATX board (that's the largest form factor). I wouldn't get anything smaller than that. I've build a couple of micro-ATX and micro-ATX systems and there's never enough room. I've used all ASUS boards. The one I'm using now is an ASUS M4A88T-V EVO/USB3.0 Also, you have to make sure your case is the same form factor as your mobo. Some cases will work with more than one form factor.
Now, as for power supplies, I know even less. It's useful to know what voltage and amperage your motherboard requires in order to POST (Power On Self Test). The voltage is really standardized, that will be easy because they're all the same. The problem for me was amperage. This was very difficult to find out for my board, and I don't think the answer that I got from ASUS was correct. What I would recommend is getting a power supply with modular cables from a name-brand manufacturer that has all the juice (watts) you need for your computer. If it doesn't work, then you need to start looking at the current (amperage). If you get to that point, it's going to be a long haul because 90% of all PSU makers deliver the same amperage on the same rails. It's going to take you a while to hunt down one of those 10% that delivers what you need. I finally found out that the Antec TP-750 will give me what I need for my machine. It has a higher amperage than any of the other PSUs I've tried. It's not modular so it might be harder to install, but I'll have to make do.
If you can order online it will be cheaper and you'll be able to get what you want. I highly recommend Newegg.com.
I know that's long. I hope it helps. I'll keep checking back.