You don't need to tell us your budget. XD
Easily I'd pick the two GTX 460's, They scale very well and reviews have been done, the scaling on the 460 is so good it basically scales 100%. Also the power consumption of the two 460's is even less than one 480 on load. Temps will also be lower allowing for an even high OC. The main thing is that I wanted to tell you, although Zotac is a good company and all, the only one I'd get is the GTX 480 version.
For the GTX 460, I'd choose the ENGTX460 ASUS DIRECTCU TOP 768gb. Since Asus really perfected the GPU cooling with the DirectCu and it easily OC's with load temps being EXTREMELY low on furmark. Check this review out! http://www.legitreviews.com/article/1369/17/
P.S. The reason this card is good, it's only 175$ on Newegg (assuming your in the US or CA) When you SLI, the total cost will only be 450$. Much cheaper than the GTX 480 for better performance.
Also if you want 1gb really bad, the best option is the MSI GTX 460 Hawk 1GB. Very good cooler, fixed the Twin Frozr fail design and ended up with a much better cooler and card. Plus it's cheaper than the zotac and IMO a better card. It's 20$ cheaper than the AMP model.
Seriously though, if the Asus model didn't impress you, check these OC's and Load temps out on this card.
Just checked the review on the Zotac AMP, 10*C hotter than the HAWK with less FPS, therefore the same reviewer was not able to OC as well as the HAWK model.
The GTX 460 SLI mopped the floor with the GTX 480 (single) in the 3DMark Vantage at a cheaper price... So that really justifies getting the GTX 460. Plus power consumption is basically the same for both on load, but the SLI 460's gets way better performance. SEE review below. Also look at the overclocking, you can OC the Hawk ver. very far and that'll be an even better performance. Plus many games favor dual card rather than single, so you'll see better performance with the two 460s.http://www.guru3d.com/article/geforce-gtx-460-sli-review/13