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The issues you are describing don't sound like a hardware bottleneck. Sounds more like a configuration problem. Possibly some driver or another isn't playing perfect with everything else? Some config/install not correct w/ Win7 & it's having brain farts? Some application(s) you have running are seemingly in conflict.
I always blame Windows. Just because ... it's always doing something completely retarded in the background and doesn't feel that you, as the owner, have any rights to know what cracksmoking fuckheaded thing it's trying to FAIL at.
That aside, your upgrade options are limited. Still don't know your budget, but ...
RAM - 6 GB? That's not a typo? Which would mean you have a 2 GB plus a 4 GB chip. To get maximum efficiency out of duel channel memory, you really want a matched pair.
Your mobo tops out at DDR3 1600 memory.
So, let's assume for the moment that you might want to and have capability in your BIOS to overclock. Under the best of conditions you could plop in 2 x 4 GB matched set of DDR3 1833 chips and push your timings & clock as best you can. That will not be a huge performance tweak, but it's certainly not zero. By itself, nothing to notice, in combination with other tweaks, it could add up. Good 1833 chips that can be pushed can be had for $40-$45, and you'll want timings that look like 9-9-9-27 or under; if you only run at 1600, you want to see 8-8-8-24 or under ... and those are optimistic numbers to shoot for to make the money & time worth it.
SSD/HDD - you are limited to SATA II / 3.0 GB/sec speeds. Getting a top-performing SATA III SSD will certainly maximize what performance is possible, but the top end of that capacity will be outside of your reach. A RAID 0 of a pair of SSDs (SATA II or III) can generate some impressive numbers .. but, you're gonna want to
want it 'cause that will take time & money. Might be worth the investment from the standpoint that it will be good training for you and you could migrate those drives to another mobo if you decide to go with a Full Monty upgrade within ~18 months.
Also, when SSDs (especially 'older' ones from last year) get really full, they tend to gum up. Rule of thumb has been to keep ~10% of the SSD free. So, @64 GB, your formatted capacity should be right around 58-ish GB ... does your SSD have at least 7 GB of unused space on it? Might not hurt if you can keep 8 or 10 free. If you're down to 3 or 2, that could be (part of) your observed problems.
CPU - I've already said a lot on that. If you can still find an 1100T and you want to do some overclocking, go for it. You could get a very nice performance bump out of that ... especially if you upgrade the RAM and aggressively tweak it at the same time.
Cooling block - don't know what you're using now. If it's the stock block that came on the Deneb 965, it has it's limits & you'll want to upgrade. Again, only worth it if you want to get all geeky and overclock. If you go that route, budget for a few good case fans and make sure they are assisting your cooling block.
PSU - overpowered for what you can do already. Be happy with it.
GPU - turns out your original supposition is correct: about the best thing you can do, especially in light of the game load you intend, is to upgrade your GPU. Others here are more up to those recommendations than I am. I'm not a gamer and am really happy with my nvidea GTX460. It does far more than I require and it's highly linux compatible, as well as overclock friendly. But, you need something more cutting edge & I'm not up on the current tech there.
Set yourself a budget and shop hard for the best GPU you can get for it.
mobo - You are reaching the limits of what your mobo can do for you, so probably start planning to upgrade the whole rig from scratch in 12-24 months, depending on what this one can do for your needs. No telling what tech will be available in 18 months, so just be zen and keep your finger on the pulse so you'll be positioned to buy when you're ready to move on.
Hmmm. Speaking of resource conflicts ... any chance that your on-board GPU isn't disabled properly and there's some idiot conflict with your PCIe GPU ..?
All that said ... I think you need to look to how your operating system is installed and configured, what options you do & don't have enabled ... and look to what you have installed. Somthin' Ain't Right, based upon your performance report. Figure out what that is and you'll have an upgrade.