For a single card/GPU the 580 is the fastest no question. With that 850w PSU you can buy a single 580 now for great performance and SLI later no problem when you need more power. You could also go with 2x 6950 2gb in xfire or 2x 560 ti 2gb in SLI for maximum performance now, but less upgradeable in the future.
At 1024x764 a single gtx 580 is probably more than enough for 3D and phyX (just make sure your CRT supports 120Hz though). At 1920x1080 2 GTX 580's in SLI would be much better for 3D, especially if you want to max games like Metro 2033 in 3D. If you want to upgrade your monitor now and still be 3D capable, keep in mind that 120Hz 1080p LCD's are still pretty expensive.
Well, for 1024x764, a single 560 is more than enough, no need go higher than that unless you are planning to get a new/bigger monitor...
Actually, your whole PC is just too overkill for just a single 1024x764 monitor.
Response time is 5ms, so it's not the best for gaming but it seems like a good price.
To be honest though, if you are used to gaming a lot on a CRT with 80Hz+ refresh rate, then switching to a 60Hz 5ms+ response time LCD will be quite different, at least for me it was. Switching from a CRT to a 60Hz 2ms LCD, I noticed a lot of tearing in games where I get 100fps or more, but at least the 2ms response time meant no ghosting issues. Eventually I got used to it, but in hindsight I wished I had gotten a 120Hz monitor. If you want true CRT-like smooth gaming you really do want a 120Hz 2ms LCD monitor, but unfortunately they are still quite expensive even today, which is why many hardcore gamers still use CRT's.
my crt can play perfectly i don't want to give it up as much as i can i don't like AA and my crt has survived a throw to the ground it is 10 years old and still running actually it will do anything i throw at it even HD netflix
5 years is forever in technology so who knows if it will be enough for that long, but refer to my first post on what you should get. I personally like single card solutions just so I don't have to deal with SLI/xfire issues such as PCIe spacing, top card getting to hot, driver issues (although less common these days), etc. So my vote is for a single GTX 580 now, and get a second 580 later if you need more power in a couple years. But, again, if you want max performance now 2x 6950 2gb or 2x 560 ti are both faster than a single 580.