5 Year warranty sounds good. In my experience though, Zotac cards can run quite hot and sometimes the cooler is defect. This is only in my experience though, but when I have seen it on 9 cards and 6 of them being Zotac x70 or 465, I stop trusting. 2 others were reference coolers (3870x2 and 4870). This doesn't mean that Zotac doesn't have quality cards of course, but for me they have wasted their chance of ever selling me a gpu. I would rather clock a card from MSI, ASUS or GIGABYTE to achieve the same as the amp card.
The 750 is enough for SLI and considering your other parts a 650W would cover this too. The reason I say rather go for HX or AX is explained in this review
http://www.hardwaresecrets.com/article/505 , I know it isn't anandtech or tom's hardware, but as a PC builder I can second their notion on cable management.
And to everyone who doesn't think it matters: It really, really does matter if you have good cable management or not. Dust will gather up more easily if the cable management is bad and the chance of dangerously high temperatures or even fire will dramatically increase.
As for the screen, I am running a 1080p 27" atm and it will get changed to a screen with 1440p or higher. It can be troublesome not to get annoyed by "low resolution" in certain games. (crysis 2, AC3, BF3) But I would still choose a 27" 1080p to a 24" 1080p any day.
TL;DR
Switch your PSU to a HX650 or a AX650, it is sufficient to run 2 670 in SLI and your cable management will be 200 times better.