Doing a reasonable upgrade requires that the original build be set up for that eventual upgrade. And even then, the money is oft "better spent" doing it from the getgo.
1. For example, if you drop from 2 x 8Gb to 2 x 4GB for RAM, yes ... you can add a 2nd set of 2 x 4B but you run the risk of compatibility (even w/ exact same model)and may have to increase voltage / reduce timings. Also the CPU OC may not remain stable with 4 modules.
2. PSU - You might spend just $10 buying an SLI capable PSU.... so spending $80 for a 600 when the 750 watter is $90 as an upgrade, means a total investment of $90 for the upgrade versus jusr $10 if preparing for the upgrade on the original build. I wouldn't use either of the two PSUs that you have.
3. MoBo - This is what the system is built on .... skimping here on the original build I think is unwise as if you change that, it means wiping the OS and starting fresh. A Z series OC capable MoBo will bring you overclocking, SLI and a better audio solution for a small increase in investment.
4. GFX Cards - For the 1st time two nVidia x60s didn't outperform the x80. Two 960s don't even equal the 970.
5. Storage - The HD is about 40% as fast as what is currently available. Given the above, I think your best bet is to sell the complete system and add the income to the $800 budget.
6. Cooling - Liquid Cooling ... what kind ? Custom Liquid Loops are a wonderful thing, great for reducing system noise to inaudible levels. If you mean CLC type liquid cooling, it's a complete waste of time.
a) CLCs don't beat air coolers... H100i is 12 times louder and runs 2C hotter than a Noctua NH-D15 air cooler. The use of cheap aluminum rads means that to even get close to a decent air cooler, the fans must use an rpm that makes them scream.
b) Overclocking stops when you exceed either thermal or voltage loads. With current Intel CPUs and nVidia cards, you are just as likely to hit the voltage wall before you hit the thermal wall with a decent air cooler.
c) Most people w/ CLCs ignore the radiator manufacturers recommendations and exhaust radiator air out of the case. Rad air should always blow in... without exception. And while common sense might cause you to think that the warm air inside the case will be an issue, mounting some temperature probes will prove otherwise. In addition, I have yet to see a component with a warning label, telling me that 28C air is a bad thing and causes an issue for any component. And the other part that also gets left out when this is offered. How is this different from an air cooler ?
The air cooler uses pre-heated warm inside the case air to cool the CPU. This obviously represents a worse condition that using outside ambient air. The air cooler's 2 fans push that hot air out into the case ... how come that's not a bad thing for the components ? Example:
Ambient Air = 23C
Case Air = 28C
-The air cooler starts with 28C intake air and therefore will have less cooling efficiency than the rad with 23C
-The CPU makes the same amount of heat so the air temp inside the case regardless of what method is used.
-The air cooler's 2 fans do not change air flow in and out of the case. Rad however greatly increases the air flow thru the case; the radiators 2 fans do which means you have higher air turnover.
So while liquid cooling can offer significant advantages, increased performance with modern CPUs / GPUs will bring at best small increases in performance. Custom Loops or OLCs like the Swiftech H40 X2 will however bring better temps and much lower noise than CLCs. Just do your reasearch and make sure you are getting what you think you are getting. See results at 17:12 here:
https://www.youtube.com/watch?v=TivNOgQqW-M
So given what you have, I just don't see anything that's "worth keeping".
I'll put together a build for you in few minutes