Trying to figure out where to go from here

I discovered the hard part about building a custom rig, getting the $1500-$2000 set aside with my wife turning into a new purse. So I broke down and picked up a custom from a guy I found on Craigslist. Here's what I got.

Intel Core i5-2500K: http://www.newegg.com/Product/Prod [...] 6819115072
GIGABYTE GA-Z68MX-UD2H-B3 MoBo: http://www.newegg.com/Product/Prod [...] 813128503R
8gb Patriot DDR3 1600Mhz: http://www.newegg.com/Product/Prod [...] 6820220569
Western Digital 500gb 7200rpm: http://www.newegg.com/Product/Prod [...] 6822136143
Samsung DVD/CD Combo drive: http://www.newegg.com/Product/Prod [...] 6827151233
Winsis Wb-27 Mid-Tower case: http://www.newegg.com/Product/Prod [...] 6811234031
Breeze 500w ATX psu: http://www.directron.com/br50atxcopos.html
Windows 7 Ultimate 64-bit: http://www.newegg.com/Product/Prod [...] 6832116718

This has ended up being a get my foot in the door system, so I can upgrade a part at a time. Can't complain too much since I only paid $500 for it. I'm eventually going to an eATX MoBo (probably Maximus IV), a full tower to accommodate it, a couple video cards (probably a couple (560 ti's), high end sound card, blu-ray drive, and maybe an ssd. I'm trying to get some advice, and instead of crowding it up with a thread for each question I'll just put them here.

1. I just realized the psu is a piece of crap that needs to be replaced, I'll probably go up to 1000w. But for now, what kind of video card can I get in there without blowing it up? The guy I bought it from was pretty vague. His advice was any card under $100 would be fine.
2. There's a lot of back and forth, but ATI/AMD or Nvidia? I'm leaning towards Nvidia, but more advice never hurts.
3. If I'm only planning on 2 video cards, is eATX MoBo necessary? I was going that way because it'll run 2 cards at 16x simultaneously.
4. Last but not least, what order would everyone recommend the upgrades in?

1. I have a gtx 460 Sli setup and believe just one can draw about 300w at max load and around 500w for both.
2. Even though I used ATI for many years I switched to Nvidia because shadows were choppy on some games, so I would say Nvidia but either way is good depending on motherboard.
3. No comment on motherboard but mine has tri-Sli and found no real difference between running at 16x 16x, than 16x 8x.
4. I would recommend buying a decent psu first because 500w might be pushing it with a modern gpu.

hope this helps and I'm sure others can give you more advice

eATX isn't required for SLI/Crossfire. I have regular ATX with two AMD 5770's. Great for gaming

a 460 is a 560. 2 560tis are going to cost as much as a 7970 and its a better card, that overclocks well and you can crossfire later. Plus no messing with SLI