If I get a 1600p monitor with two 780s in SLI with an i5-2500k cpu overclocked to 4.5ghz, will my cpu bottleneck me? And if so how badly? I'd like to play on max settings in most games but can understand if a game like Crysis 3 wont be playable at max settings.

If you want to play at max settings, it should not bottleneck two 780sm especially overclocked to 4.5GHz as it is the case. Crysis 3 is not that CPu demanding game anyways, yous hould be fine.

I think you will be fine. Even if the CPU did hold you back, using dual 780s at 1600p would still be "economical" - there would still be a performance gain.

I use a single 780 at 1440p. Might be worth using a single 780 and seeing if that gives satisfactory performance. Might save you a little bit of money.

Ensure you have a good 750W PSU for dual cards.