560Ti 448 & older 550w PSU = Skyrim crashing?

So I recently purchased a second hand MSI 560 ti 448 Frozr and I love it, it's a huge upgrade from my previous HD4870 and up untill recently I haven't had a single problem. I played Mass Effect 3 with zero problems at high settings and great frame rate, I was able to play Super Street Fighter IV Arcade Edition completely maxed out at highest settings with 120fps, and even Kingdoms of Amalur with a steady FPS highest settings.

So here's where I think I have a problem: I originally started playing Skyrim when it first came out and only played a few hours of it because i was forced to play it on Low/Medium settings with my old card and even then I was getting about 40 average FPS. It was playable but i wanted to experience it in better quality so i decided to wait. Since then I've added an excessive amount of memory, a solid state drive, and the new video card and have been able to play High settings around 50fps with little trouble. I am pretty limited by my CPU which is a Core 2 Quad Q9400 but it does it's job well enough and I will most likely upgrade to a new intel CPU later on this year but not right now.

TL;DR version: My new 560 ti 448 seems to be pushing the GPU useage when playing Skyrim as my MSI afterburner monitor shows it using up 90+% when there's alot going on. Due to this I've had my computer completely power down on me twice so far. No blue screen, no CTD, simply powers down.

I'm looking for experienced opinions but I'm pretty sure my problem is simply that i have the bare minimum PSU for this graphic card, and when it's running up the GPU useage too much i simply don't have enough power and it crashes my system. I've tried other games to see how much load it's putting on the GPU and nothing has come close to Skyrim yet (yes it's also heavily modded) but it was giving me great performence.

Antec Trupower II 550w PSU (says 530w effective on the side)
Core 2 Quad Q9400 stock @ 2.66 ghz
Corsair XMS3 10333 4x4gb: 16gb RAM
MSI Frozr III 560 Ti 448
Crucial M4 64gb SSD
Assorted WD, Seagate 2x 160 gbs, 1x 300 gb
DVD+/-RW drive


Is it pretty safe to say I don't have enough juice to push this card? I've ran CPU stress tests and it doesn't look like it's overheating, I've run the GPU stress tests and they completed with no errors, but i think running at 90%+ load for extended periods of time are crashing me. Any advice / opinions as to best way to further test this without possibly bricking my computer are greatly appreciated. As well as recommendations on how big of a new power supply i might need. Thank you