Antec builds solid cases, that one should be fine. I have an Antec P182- despite being a little heavy, it's a great case.
"SLI Ready" is a term that's used pretty loosely when it comes to PSU's. All it means is that there are 2 PCI-E power connectors instead of 1. Doesn't matter if you're using ATI or NVIDIA because they all use the same power. Just be sure that you have the right motherboard if you're considering Crossfire. I've always been in favor of buying the most powerful PSU you're willing to spend money on up front. Who knows what you might want to add in the future? That OCZ you linked to is solid, I'm very happy with mine. Shouldn't be overkill at all.
From personal experience with a Radeon HD 4850, I can say it's an excellent performer for the price (heh, I sound like I'm doing an infomercial). It's cheap enough that it probably wouldn't be a bad idea to pick up a second one down the road.
As far as CPU/Motherboard, I'm not too familiar with AMD but it looks like the Phenom II 720 is a great bang for the buck. Good luck with your build!
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