Update:
8GB RAM 4x2GB DDR3 Mushkin Radioactive 996974 7-9-7 24 timings at 1600mhz tried al 4ram's or 2 by 2 pairs black slots only, grey only, all (rotating pairs and rotating individual) 1.5v vdd, 1.65v (specs), 1.7v (overvolt) NB 1.4v SB 1.65v or 1.7v (overvolt), 1333mhz, 9-9-9-24 1T Auto Linked P1,P2 Auto, 1600mhz, 9-9-9-24 1T Auto Linked P1,P2 Auto, 9-11-11-29 2T Auto Linked,  P1,P2 Auto, Manual 9-11-9-29 2T Unlinked Linked,  P1,P2 Auto, and in some config only 6GB visible lately, one stick appears dead now (must test it on other platform ), with this stick MemTest 4.3.7 option 1 in Menu a lot of errors..., Tried Many Vista SP2 Ultimate 64bits: Apr 2017 upd incl, or till 2013 upd incl (older copy), from dvd or usb stick (usb2.0, usb3.0 stiks) Copy Files > ok , Expanding very slow ~1h, Install features slow, Install updates terrible slow 1-11/2 h,  Configuring installation slow 1h>>>, reboot, black screen Configuring installation slow 1h>>>, green screen slow 30min rolling circle, green install screen Configuring installation slow 1h>>> stuck (progress bar stuck in the same position always ...), forced shutdown or reboot, HDD System 500GB (Seagate Server Ed, NO Realoc Sect Count, 100% healthy via HDD Low Level Format Tool 4.x, Ashampoo HDD 3 Control, etc. anyway SYSTEM with GB EP45-DQ6 which EVGA MB replaced as upgrade worked from 2008 without reinstall Vista Ultimate SP1 upd to SP2 64bits and XP Pro SP2 64bits, till some power spikes via outages by E-ON provider which botched BIOS ...) tried all space, different partition sizes at beginning or end, in latest config tested corrupted 2 other HDD mounted which now want chkdsk which started freezes, Tried Win 8 Pro 64 bits also which installs apparently (wanted this to be 2nd OS though)  >>>>> dropped Mushkin's (appears to damaged 1 stick in process, I know all worked in other Phenom BE 965 + GB 770TE-UD3 MB out of the box at 1600mhz, only 7-9-7 24 timings had to input there manually... :(  
 
Acquired 8GB RAM 4x2GB DDR3 Kingston Limited Ed KHX1600c9D3BK2/4G, 1.7-1.9v VDD (high voltage generation ddr3),  installed Unlinked, P1, P2 Auto, NB 1.4v SB 1.7v (overvolt), 9-9-9-24 1T at vdd 1.7v, 1.8v Memtest+ stable, P1, P2 Enabled, NB 1.4v SB 1.7v (overvolt), 9-11-11-29 2T at vdd 1.8v, 1.9v Memtest+ stable  Tried Many Vista SP2 Ultimate 64bits: Apr 2017 upd incl, or till 2013 upd incl older copy, from dvd or usb stick (2.0, 3.0) Copy Files > ok , Expanding very slow ~1h, Install features slow, Install updates terrible slow 1-11/2 h,  Configuring installation slow 1h>>>, reboot, black screen Configuring installation slow 1h>>>, green screen slow 30min rolling circle, green install screen Configuring installation slow 1h>>> stuck (progress bar stuck in the same position always ...), forced shutdown or reboot, HDD System 500GB tried all space, different partition sizes at beginning or end, the corrupted 2 other HDD mounted which now want chkdsk which started freezes, disconnected them still same behave , also forgotten to say BIOS FDD controller > disabled, FDD > None, SpeedStep Disabled ...
I will try tomorrow Win 8 Pro again, and renounce to Vista which seems to hate this MB, RAMs, this MB is a nightmare in terms of DDR3 sticks...
 
Any suggestion's folks ???
 

Welcome to the forums PCgamer911.

For now I suggest getting MSI Afterburner where you can see (in the Hardware Monitor) how much system RAM and video RAM you're actually using, as well as CPU and GPU usage while gaming and go from there..No sense in trying to get more system RAM to work if you're not using all, or nearly all of the 8GB you have now..Reply back with your findings.

@PCgamer911..Doesn't appear to be running out of system RAM but more might help, hard to say..What about VRAM usage ('Memory usage, MB' in Afterburner, and EVGA Precision)?..Running all that other stuff in the background won't help anything either.

Another really good utility to use for checking on things is AIDA64 Extreme (30-day trial, or pay)..You can set it up to log a lot of memory-related and other stuff.
 
 

@PCgamer911..People have gotten 16GB to run in a 790i, but it's hit/miss..The 790i is very picky about RAM.
 
The 790i doesn't like low(er) voltage RAM very much, the FTW that I had wouldn't even boot on Auto Memory Voltage with some OCZ stuff at it's SPD of 1.5V (2.0V spec)..Had to put in Patriot Vipers (the 1600MHz 7-7-7-20 green ones that are to kill for) to get into BIOS and manually set it then put the OCZ stuff back in.

Increasing Memory and SPP voltages a little is often neccessary along with manually setting the timings and Command Per Clock (CMD) to 2T and maybe enabling P1 and P2 and increasing tREF a little to get 16GB working..Can be a pain..Can also be futile.
 
You can search for some 790i 16GB success stories like here and here and here..If you flash to the XFX BIOS you'll lose VDoop Control, CPU PLL and probably a few other things..Don't know if I would do that..Well, yeah I would 'cause I have two other PCs and could reflash or recover from a bad flash via USB flash drive and the BIOS chip is replaceable and can still be found on eBay and other places and EVGA prolly even still has some tucked away somewhere you could buy from them if it's ever neccessary.

Good luck!