Hmm, that Prime95 error is odd. I've gotten the same error while trying to overclock too high (running Prime95 for even 5 minutes can tell you if you overclocked too high for stability).
If you got $2k to spend, then by all means do it. I would pesonally try and troubleshoot your current system a little, since for a few hours of your time you could find the issue. But if you have other reasons to upgrade like your current system wasn't up to snuff even if it was working properly, then go ahead and do it if you got the cash. I'm sure you'll have a blast with your new system.
BTW, those voltages sound okay. And your PSU is better than 95% of the PSUs we have here. Unless its somehow defective, it shouldn't cause any problems. I've seen fairly intense systems running on cheap 300-350 W PSUs without issue. Don't pay much attention, if any, to the negative voltages. They are seldom used and almost always not even close. Also, your 11.43 V on the +12 V is perfectly normal. That's about 5% off, which is about as good as you can expect the majority of PSUs to regulate at.
edit: And no, absolutely not about the ATI card. THey are JUST AS GOOD as Nvidia, usually its other components causing problems. My previous two systems had ATI Radeon cards and were stable as ever. Now I'm running Nvidia for the first time ever. I prefer the Nvidia driver interface to ATIs by a large margin, but as far as driver quality and stability, I consider them equal.