Upgrade Advice Request

First I would like to apologize if this is not the correct area to post this thread. I would be more than happy to take down and/or re-post this in a more appropriate thread if requested.

I don't feel like I'm getting the most out of my system. Like there is a huge bottleneck somewhere and I just don't have the knowledge needed to find it myself. My current system looks as such:

OS: Windows 7 Ultimate 64bit
MOBO: MSI 890GXM AM3 AMD 890GX SATA 6Gb/s USB 3.0 HDMI Micro ATX AMD Motherboard
CPU: AMD Phenom II X4 965 Black Edition Deneb 3.4GHz Socket AM3 125W Quad-Core (NOT OC'ed)
RAM: G.SKILL Ripjaws Series 8GB (4 x 2Gb) 240-Pin DDR3 1600
GRAPHICS: PNY GeForce GTX 460 (Fermi) 1GB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready SLI Support
HDD: Seagate Barracuda 7200 1TB 32MB Cache SATA 3.0Gb/s
PSU: Thermaltake TR2RX-550 ATX 12V 2.2
CASE: Antec Nine Hundred Two V3 Black Steel ATX Mid Tower Gaming Case
COOLING: Corsair CWCH50 Water Cooler w/ 2x120mm (push-pull)

1) My immediate thought's bring me to getting another GTX 460 and running in SLI (which would require a new PSU for sure).

2) I get drastic slow downs when using shadows at anything above medium shadow quality in any game. Again, pointing to a new, or SLI'ed graphics card.

3) My PSU is fairly old. Is there a chance that with my set up, I'm placing strain on my current graphics card or processor? I seem to only get BSOD's when playing Assassin's Creed, and from what I've read all of the AI processes are very intense on the processor. On a second note the BSOD's occur in 'smokey' or 'foggy' areas. So logically I figure the AI strain on the CPU coupled with a graphics card that already seems to have shading difficulties pulls a lot from the PSU thus throwing a BSOD when the required power needs aren't met.

Any other suggestions or idea's outside my own suspicions or confirming them would be greatly appreciated. Where should my next upgrades lay? I've budgeted roughly $200-$300 for my next parts (+/- $50).

I just feel like I should be getting a lot more out of my system than what I receive. I would be happy to run any bench marking at request - I've run a few benchmarks and find myself without the knowledge of how to use the information I receive back from the test.

WIN7 Index
Processor 7.5
Memory 7.6
Graphics 7.5
Gaming 7.5
Primary HDD 5.9 (Non SSD - I get that part.)