Need advice, deciding on a new GPU!

I need help choosing a GPU for my system. I have a dated 6850 that has lasted me up until this point! I have built a new rig a few months ago, and I think it's about time I updated from my old card. Here is my current set up!

CPU: Intel Core i7-3770 Ivy Bridge 3.4GHz
PSU: 850w 80 Plus Silver. (It's quite old, probably five years. But seems to be holding up!)
Memory: G.SKILL Ripjaws Z Series 32GB (4 x 8GB)
Hard Disks: 125 SSD (OS, and a few games/applications), 500 GB WDC(Main hard drive.), 2 TB (data storage)
Motherboard: ASRock Z77 Extreme 4
Monitor 1: 1080p, 1920x1080.
Monitor 2: 1440x900 (Mostly used when working on art, or streaming so I can see chat and my application at the same time. Usually it is always turned off.)

If you need more information, I can give! That's the main components however. :3

Right then, I'm a little torn between what GPU to upgrade to. I'm a pretty avid gamer, and also an aspiring 3D modeler (main reason for the 32gbs of RAM). So I'd like to go with something that will last me for a while! I only really use one monitor to game on, the second one is there so I can view chat, or watch videos while I'm working on art! (When I'm not being lazy, of course.) My budget is probably around the 400$ region. Though I would prefer something lower then that, though I'm always willing to spoil my baby, for that extra performance. ^-^

So far, I am leaning towards a GTX770, maybe with 4GB. I know people say the higher gigabytes are not needed, but in a few years I think it will make a difference! Especially for a multitasker.

Or, would it be worth going for a 780? I know the differences aren't that big, especially for $200~ more... I much rather go with the 770, save money, and then upgrade in two or three years!

Then, there are the new Radeon 200 series. Like the R9 280X. It seems comparable to the 770, but the price on those have gone up quite a bit the last few months! Especially the 290X.

Another option would be using two cards, but I don't know much about that, or if my system itself could handle it. Then I might just go with a 770, then buy another one when they are cheaper! Uwah, too many options. '^' Not to mention all the different companies, like Gigabyte and EVGA. I have no idea which one of those to go for either! I'm pretty clueless, even if I did build my rig by myself. XD

So yeah, those are my thoughts. I guess I just need some advice as to what direction to go for!

In the 400$ region, preferably a lil less. :3c But I'm willing to spend a bit more if it's really worth it. I'm kind of iffy about spending 200+ for slightly better clock speeds... ;-;