i have gtx560 ti OC by Zotac with Q9400... i play skyrim @1680*1050 ultra settings 4xAA 2xAF and my fps min/max 20/60
I have GTS450 Zotac with Q9650 (3.0quad), at 1920x1080 almost all high 8xaa 16xaf and fps min/max 30-60 (if I go ultra, which is damned close to high, I get about 22-60 fps)
To the original poster - I'd look into your PSU, if you have only 450watts, chances are your 12v rail is insufficient (my card needed at least 22A, I'm assuming yours is the same, and 450 seems low, I doubt it has 22A, even if it did, I think you'd need at least 500watts to make sure there's no problems).
Look on the side of your PSU (power supply unit, for those who don't know what that means), you'll see info on the voltage, look at the 12V, it should have the min required for that particular card (you'll have to find out), for me, it was 22A (amps).
EDIT: here's a link that gives the minimum for the 560 Ti (probably similar to the 560, but not certain). According to this, it's 30A on the 12V rail. I know NVidia doesn't show that info on their site, but the 12V showing on your power supply makes a difference.
http://www.tomshardware.com/forum/319846-33-understanding-power-requirements
I know some of this info is complicated :S but this link explains it pretty well. If your PSU doesn't put out sufficient amps, it can causes freezes and crashes. My card needed minimum 500watt PSU (cause the card uses something like 100W, the rest is for the system, 500W is safe to make sure you have more than enough), and I needed at least 22A on the 12V rail (that's the power that goes directly to the card, a separate power cable, mine had 6 pins). I hope this helps, and doesn't seem a little overwhelming.
btw, I may have overcomplicated it

and I don't know if I fully understand it / explained it. I do hope the info helps.