I do not like your proposed change in a number of aspects.
1. For gaming, the graphics configuration is the most important component.
5770 is now considered as a entry level gaming card.
With crossfire, you will do fairly well in fps benchmarks. But there are playability issues with dual cards. Lagging and screen tearing. I think you would do much better to plan on using a single good card.
Something like a GTX760 would be appropriate.
2. How adequate your 600w psu will be is dependent on it's quality.
Exactly what brand/model are you using?
See where it fits on this list:
http://www.eggxpert.com/forums/thread/323050.aspx
A good 600w psu will have enough +12v amps to run even a GTX780 or R9-290.
3. Yes, the duo is old, but it may well still be capable enough. Few games can use more than the 2 cores. 4 at most.
To help clarify your CPU/GPU options, run these two tests:
a) Run your games, but lower your resolution and eye candy.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
If your FPS stays the same, you are likely more cpu limited.
b) Limit your cpu, either by reducing the OC, or, in windows power management, limit the maximum cpu% to something like 70%.
This will simulate what a lack of cpu power will do.
Go to control panel/power options/change plan settings/change advanced power settings/processor power management/maximum processor state/
set to 70% and see how you do.
If you want a cpu upgrade, at your budget level, I would look at an Intel quad like the i5-4670K.
Here is one study on <$200 gaming cpu's:
http://www.tomshardware.com/reviews/gaming-processor-frame-rate-performance,3427-9.html
4. Here is my canned rant on planning for dual cards:
-----------------------------Start of rant----------------------------------------------------
Dual graphics cards vs. a good single card.
a) How good do you really need to be?
A single GTX650/ti or 7770 can give you good performance at 1920 x 1200 in most games.
A single GTX660 or 7850 will give you excellent performance at 1920 x 1200 in most games.
Even 2560 x 1600 will be good with lowered detail.
A single gtx690,7990, GTX780ti or R9-290X is about as good as it gets for a single card.
Only if you are looking at triple monitor gaming, or a 4k monitor, might sli/cf will be needed.
Even that is now changing with triple monitor support on top end cards and stronger single card solutions.
b) The costs for a single card are lower.
You require a less expensive motherboard; no need for sli/cf or multiple pci-e slots.
Even a ITX motherboard will do.
Your psu costs are less.
A GTX660 needs a 430w psu, even a GTX780 only needs a 575w psu.
When you add another card to the mix, plan on adding 200w to your psu requirements.
Even the most power hungry GTX690 only needs 620w, or a 7990 needs 700w.
Case cooling becomes more of an issue with dual cards.
That means a more expensive case with more and stronger fans.
You will also look at more noise.
c) Dual gpu's do not always render their half of the display in sync, causing microstuttering. It is an annoying effect.
The benefit of higher benchmark fps can be offset, particularly with lower tier cards.
Read this:
http://www.tomshardware.com/reviews/radeon-geforce-stutter-crossfire,2995.html
d) dual gpu support is dependent on the driver. Not all games can benefit from dual cards.
e) dual cards up front reduces your option to get another card for an upgrade. Not that I suggest you plan for that.
It will often be the case that replacing your current card with a newer gen card will offer a better upgrade path.
The high end Maxwell and amd 8000 or 9000 series are due the end of the year or next year.
-------------------------------End of rant-----------------------------------------------------------
5. I do not much like all in one liquid coolers when a good air cooler can do the job.
A liquid cooler will be expensive, noisy, less reliable, and will not cool any better
in a well ventilated case.
Liquid cooling is really air cooling, it just puts the heat exchange in a different place.
The orientation of the radiator will cause a problem.
If you orient it to take in cool air from the outside, you will cool the cpu better, but the hot air then circulates inside the case heating up the graphics card and motherboard.
If you orient it to exhaust(which I think is better) , then your cpu cooling will be less effective because it uses pre heated case air.
And... I have read too many tales of woe when a liquid cooler leaks.
google "H100 leak"
6. I like 16gb of ram, but buy a 2 x 8gb kit. It will usually be less expensive. a 4 stick kit is harder to match and harder for a motherboard to manage.
Fancy heat spreaders are not needed, they are mostly marketing. Low profile ram is fine.
7. I will never again build without using a SSD for the OS.
It just makes everything you do so much quicker.
A one hour widows update will take 15 minutes.
Files open instantly.
A 120gb ssd will hold the OS and a half dozen games.
Consider deferring on the hard drive, you can always add one later.
With a 240gb ssd, you may never need a hard drive.
I like Samsung EVO for the ssd.
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