That's fine, then. If you anticipate ever going crazy with graphics cards and having two ridiculously powerful ones, go with this one for PCI bandwidth: http://www.newegg.com/Product/Product.aspx?Item=N82E16813157271
Case size does not affect the PSU. Everything but servers and some OEM builds uses ATX-standard PSUs.
How much are you actually willing to spend on this build? It would be good if you set yourself a ceiling. If you can fit in a nicer graphics card, I don't think you'll regret it.